Well, there are lots of tips but I've tried to cover the basics here -

Choose between third person limited, objective and omniscient

In third person narration, the predominant pronouns describing the action of the story are ‘he’, ‘she’, and ‘they’. Third person narration may be ‘limited’, ‘objective/uninvolved’ or ‘omniscient’.

‘Limited’ third person narration isn’t told directly by the viewpoint character (there is no ‘I’ telling the story). Yet in limited third person we still see the story from the character’s perspective, even though the narrator stands outside the character, describing their actions.

Ursula Le Guin describes how this type of third person narration is ‘limited’ concisely:

‘Only what the viewpoint character knows, feels, perceives, thinks, guesses, hopes, remembers, etc., can be told. The reader can infer what other people feel and are only from what the viewpoint character observes of their behaviour.’

Le Guin notes how third person limited is similar to first person narration:

‘Tactically, limited third is identical to first person. It has exactly the same essential limitation: that nothing can be seen, known, or told except what the narrator sees, knows, and tells. That limitation concentrates the voice and gives apparent authenticity.’

Objective third person narration, by contrast, does not give the narrator access to characters’ private thoughts and subjective feelings. The narrator is like the lens of a camera or a fly on a wall, merely recording what characters say and do without explicitly telling the reader about characters’ private emotional worlds.

Besides limited or objective third person narration, you can start a story in omniscient third person. Omniscient is similar to limited third person in that the narrator stands outside each viewpoint character, describing their words, deeds and inner monologues. Unlike limited third, however, in omniscient narration you can switch between characters’ viewpoints, even within a scene. Using this type of narration, you can describe a room of a home or a landscape even when there is no character present to observe it, too.

Choosing a type of third person narration for your novel beginning will depend on the structure and ensemble in your first scene. Omniscient narration is effective when there are multiple, equally important characters present at the start of the story (such as a band of adventurers in a fantasy novel). Here, omniscient narration enables you to show how different characters feel. This multi-voiced narration is useful because you can develop multiple strong characters who each have their own individual arcs that unfold simultaneously.

Alternatively, if you have a central protagonist who is the star of the story, limited third works well. Telling the story via a single, strong consciousness helps create connection between the reader and your main character.

Begin with character action and description that raises questions

New authors often start third person stories with extensive back story and character sketching. There’s no unbreakable rule that you can’t ever do this. The danger, however, is that the beginning of the book will feel very obviously introductory. It shouts ‘I will now introduce you to my characters.’

You should show the reader a character and tell them ‘here is what makes this character unique.’ Yet you could also show a character doing or saying something that creates interesting questions. The latter option helps to avoid the sense of an info dump. Ideally, your third person opening will introduce the reader to a specific scenario that helps readers see how your character’s situation promises further revelation, excitement or high drama.

In a third person scene opening, it helps to think about your character’s immediate goal(s) for the scene as well as long-range goals. For example, you could describe a character racing to get to a crucial exam venue. This is the ‘scene goal’. The ‘arc goal’ (contributed to by the ‘scene goal’) could be your character’s ultimate career objective.

In addition to creating interest in characters’ actions, approach introductory description in third person with a light touch:

Avoid introductory character descriptions that read as lists

When introducing characters in third person, it’s easy to go overboard on itemized character descriptions. ‘Jules was five feet tall and wore her hair in a ponytail. She had a loud laugh that frightened away birds and only hated two things: gym class and small, yappy dogs.’

This isn’t necessarily ‘wrong’ yet you could show many of these details in the course of the story rather than tell them all upfront. It’s easier to get lost in a story when we’re allowed to see character detail emerge alongside story events. Where possible, reveal character description in relation to the immediate action in a scene.

Remember not to use dialogue attribution in third person unless necessary

In learning how to start a novel in third person, dialogue is often an excellent choice. Characters’ voices lend some of the immediacy of first person narration. What’s more, you can avoid a build-up of repetitive ‘he’ or ‘she’ pronouns.

In opening scenes in third person, it can be tempting to overuse dialogue attribution to show who is speaking. Compare these two examples:

They were sitting quietly when Jules burst out laughing. Two birds that had been pecking in rings closer to the bench took off for the nearest tree. Gary, who had jumped a little himself, eyed her sidelong.
‘What’s so funny?’ he asked.
‘I was just thinking,’ she replied, ‘about what you said during assembly this morning.’

Compare the dialogue above to the following:

… Gary, who had jumped a little himself, eyed her sidelong.
‘What’s so funny?’
‘I was just thinking about what you said in assembly this morning.’

The latter dialogue example is preferable. The words ‘he asked’ and ‘she replied’ are unnecessary. It’s clear from the question mark and the context that Gary is asking a question related to Jules’ outburst and it’s similarly clear it is Jules speaking in the reply.

When you begin a story in third person, remember that you don’t have to constantly remind the reader that there is a ‘he’ or a ‘she’ uttering each line. Instead, attribute statements by making characters address each other and by using context – surrounding actions and gestures.

Balance introducing character and setting

At the start of a third person narrative, it can be tempting to describe a character’s internal monologue exhaustively. Sometimes this results in thin scene setting. So much of the narrative focuses on characters’ feelings and plans that a sense of place is scant.

Alternatively, the freeing element of writing in third person can have the opposite effect. Since sticking to a single character’s perspective isn’t a constraint, you might give pages of introductory scene description.

Developing scene setting through character actions is an effective way to introduce characters and balance setting with character description. For example, compare the following:

‘The room was rubble, the scene of a colourful wrecking. Building blocks covered the floor. She would have to tidy it all up.’

‘She picked her way through the colourful rubble, surveying the aftermath. A bright yellow block jabbed her in the arch of her foot. She would have to tidy it all up.’

In the latter, we get the sense of a child’s play room as setting and a sense of the character’s weariness and physical presence at the same time. This balancing of setting and character is especially important at the start of a story where you are establishing your fictional world and its inhabitants.

Don’t make the narrator’s voice too intrusive

Starting a novel in first person is all about intrusive narration. The narrating ‘I’ is giving us access to their innermost thoughts and impressions. In third person narration, though, the narrator should be only lightly felt.

There are exceptions to every rule, however. You might choose to use a third person narrator who addresses the reader directly deliberately as a device. Still, if you want to create a sense of realism, it’s important to not make your narrator self-aware.

Learn from great examples of third person story openings

There is no single ‘right’ way to start a story in first person. Reading examples by respected authors will help you gain a clearer sense of available approaches, though.

George Eliot, for example, in the classic novel Middlemarch, flouts tip number 3. Her introductory third person character description is fairly list-like:

‘Miss Brooke had that kind of beauty which seems to be thrown into relief by poor dress. Her hand and wrist were so finely formed that she could wear sleeves not less bare of style than those in which the Blessed Virgin appeared to Italian painters; and her profile as well as her stature and bearing seemed to gain the more dignity from her plain garments.’

Eliot’s story beginning is still effective as she moves from the general to the particular, which has the descriptive effect of zooming in on Dorothea Brooke with a telescope. At first Eliot describes Dorothea as having natural beauty plain clothes amplify. Eliot progresses to describe details, comparing Dorothea’s hands and wrists to details from paintings by Italian masters. Through all this, the reader forms a vivid impression of Dorothea, even before the character speaks.

Modern readers might be more impatient with lengthy descriptions. Here is an example of a story beginning in third person that cuts straight to the action, from Hilary Mantel’s acclaimed historical novel Wolf Hall:

‘So now get up.
Felled, dazed, silent, he has fallen; knocked full length on the cobbles of the yard. His head turns sideways; his eyes are turned towards the gate, as if someone might arrive to help him out. One blow, properly placed, could kill him now.’

Mantel’s opening fits the advice under heading number 5 above. It balances character description and setting. A sense of the character being in a dire predicament unfolds together with a keen sense of place – the cobbled yard. The overall effect is to make the scene vivid. Mantel also does not over-rely on the pronoun ‘he’. Instead, she creates strong sentences using active adjectives (‘felled, dazed, silent’) and varied sentence subjects.

Read through third person story openings and write down your own observations about what they do effectively. Ask what questions they raise, what story developments they promise, and how the writer uses third person.