Writing the first chapter in a new book is . . . very difficult. Even if you've written a lot of books. Why is it so hard? Well that's a simple question that I have the answer to.
You see people go to a bookstore, pick up a book when they see a cool cover, read the back, and if they don't like how it sounds they put it back on the shelf. But if they do like it, they read the first page of the book, or the first chapter (depending on how long the buyer is willing to stay in the bookstore).
The first chapter is important, but not as important as the first page. Every time I start a new book I think about how I want the first pages to sound, and what I want to happen. Basically how I want the whole thing to start out.
If you think literary agents and editors are harsh critics about your book, then you don't know readers. There are very picky readers out there, and I'm one of them. I hate it, hate it, when someone buys me a book. Because most of the time they just pick a book they would read, and not what I would read.
Anyway, that first chapter is hard to write because you want it to sound right, and you don't just want to jump into the story. In my last post I talked about the 'staircase of chapters' take that advice. You should start out slow, but keep it a little interesting and don't try to tell a whole lot in the first pages of the chapter. You don't want the reader to feel like the book is rushed.
Say the sentence aloud before you write it, if it sounds right to you go ahead and put it on paper, if not you should think about how you can redo it to make it sound better. Think about how you would say stuff. If it doesn't sound right, and it isn't something you really would say (if you were writing about someone your age that is, because adults write books about people of all ages) then change it.
But remember, that first page in the first chapter is important! It sucks the reader in. It's supposed to make the reader want to read more, and to want to buy it on the spot. But it also is the beginning of your story, so you have to be careful.
You can't describe a character one way and then a few chapters later describe that same character a different way. The reader would be confused. What I do is write the name of the character on a piece of paper and then describe how this person looks, so that way I won't get confused.