Yes, I totally ripped off this title from Weird Al, so in all fairness here's his video. This, by the way, should be required viewing before you're allowed to publish anything in a public space.

https://youtu.be/8Gv0H-vPoDc

I'm not going to fixate on specifics here, but I want to cover a lot of ground quickly, so I'll review some of the most common mistakes new (or lazy) writers make.

WTF are you talking about?

An antecedent is what a pronoun points to, the word that establishes what that pronoun is. Consider the following:

     Hubert glared at Rasputin. He was eating his sandwich.

Apart from being uninteresting, the pronouns are confusing. Is Hubert eating his own sandwich? Is Rasputin? Or is Rasputin heating Hubert's or vice versa? There isn't a quick fix; the sentence requires a rewrite, or perhaps a supporting sentence to establish who is eating whose sandwich. Never use a pronoun without a clear antecedent.

Bad beginnings and endings

If you're unaware, a preposition is a word that describes the relationship of one thing to another, such as "on the table," or "after the party." We're frequently told to never end a sentence with a preposition, which isn't entirely true, BUT it's a rule you should stick to unless you know for certain that you can, or should, break it. In general, informal language, especially dialog, is given a pass.

     Rasputin knew good playwrights were hard to come by.

The purist in me doesn't like it, but it's not technically incorrect. I've also heard people say you shouldn't begin a sentence with one either, but that's bollocks.

   Once Rasputin left, Hubert felt free to indulge his inner ballerino.

This is absolutely correct. You should not, however, ever begin a sentence with a conjunction, and this includes the expanded list. A common offender is the word "also."

     Also, Hubert attempted his best glissade.

Totally cringeworthy, totally common. Please stop.

Use the right friggin' words

I don't care if you think people know what you mean, we outgrew grunting and pointing at things for a reason. Words have specific meanings, not just the one that's convenient to you at the time, and it's not appropriate to pick something that's "close enough" just to demonstrate a large vocabulary or break up your text.

I see this most often in people who rely heavily on a thesaurus. They believe that because a word is synonymous, it is also interchangeable. Guilt is not the same thing as shame, obtuse is not the same as abstruse, and despite the title of this book, well is not the same as good. If you're going to use a thesaurus, keep a dictionary close by.

In the same vein, be careful with homophones, or words that sound the same, or even similar. Consider accept and except. These are very nearly opposites, and you don't want to get them mixed up. Other common misuses include affect and effect, then and than, lie and lay, complement and compliment, verses and versus, and my gosh this goes on forever and then some. 

Finally, please, PLEASE make sure your verbs agree with your subject. I'm talking about simple stuff like "is" versus "are."

Rasputin's goal is to become known for his wit.
This is correct; the singular verb, "is," refers to the singular subject, "goal."

One of Rasputin's goals is to become known for his wit.
This is also correct. The singular verb, "is," refers to the singular noun, "one." This is why it's important to know how to identify the subject of your sentence.

Rasputin's goals is to become known...
I'm not even going to finish that, if you can't tell it's wrong you're beyond my ability to influence. 

One last issue, make sure your pronouns match the nouns they're describing. "They" and "their" are plural, and can't be substituted for "he/her" and "his/hers." I am aware of the whole gender argument, but I'm telling you to be aware that it's wrong, and if you choose to use it anyway at least you're making an informed decision... to be wrong... just saying.

Apostrophe catastrophe

This isn't that hard to grasp. An apostrophe always indicates possession or a contraction, and in every case where the two might be confused, the contraction will always take precedence.

"Its" versus "it's" is the one you'll encounter most frequently. You'll only ever use "it's" when you mean "it is" and "its" without the apostrophe is a possessive pronoun.

     Anfortas' umbrella falls onto its side. He takes it with him because it's raining.

The only other time you'll see an apostrophe is when you are referring to plural numbers or letters, like, There are four s's in the word Mississippi.

In short, take caution, and use the internet for looking up something other than cats and/or boobs.