The first line of your book will be one that you may spend ages deliberating or one that you write immediately but however long it takes you- don't rush it! Rushing your writing will only end up with chapters that you aren't proud of and that aren't of the quality you want them to be. If you don't know how to start, close you eyes and try and say it in your head. Don't put loads of pressure on the first line, first chapter, first paragraph- you can always edit later.

To write a good book, you need an extensive vocabulary which means downloading a Thesaurus app or something like that would be useful. Obviously, your style of writing depends on the genre and you personally but try and keep things formal unless informality is a key part of your plot. If your story is really serious, don't use text talk or any sentences that don't fit your idea.

If you have trouble writing, look online for some guides that will be able to teach you how to improve your writing- on this website alone I've seen tons of 'How to Write' books which will all be helpful. I mean, no one is expecting you to write like a published author, but having good writing and interesting sentences will mean that your book will be appealing to read.

Structuring your work is also very important, paragraphs are key! Using a range of short and long sentences has a very nice effect in any work and understanding how to structure your book is vital.