Q: When you're writing a book, is it best to write the first chapter first?
A: Not necessarily. You can start anywhere. The first chapter can be the hardest to craft, and many writers find it easier to leave it to the end. Starting at the beginning can give you a sense of continuity, but don't spend a lot of time there because you may get new ideas as the story or discussion develops that you'll want to work into the opening. The beginning of the book often ends up vastly different from the way the author initially imagined it.
Q: Why should you generally use active voice instead of passive voice?
A: The main reason you should generally use active voice rather than passive voice is that the active voice enlivens your writing, whereas overuse of the passive voice can render it dull and lifeless.
There are two primary factors at work here:
a) The focus of a clause or sentence in active voice is on the performer of the action, and therefore on the action itself, while the focus of a passive clause or sentence is on the object affected by the action, and therefore on the outcome or result of the action.
b) The passive voice forces you to use at least one relatively empty word (some form of the verb "to be"); if you want to identify the performer of the action, you must also use the relatively empty preposition "by." Empty words sit silent in your sentences and suck the life out of your writing.
If you listen to sportscasters, you will hear all active verbs when they're describing a play in the making; later, when they're analyzing the play, you may hear a passive verb here and there. No, they don't do this on purpose; it's what we do naturally when we speak.
Note how the first example below feels like a description of the event itself, while the second feels more like an explanation after the fact:
1a) Jessie threw the ball.
1b) The ball was thrown by Jessie.
Once in a while, however, the passive voice is more appropriate. It's a matter of focus. If you want to focus on the outcome or result of an action rather than on the action or the performer of the action, use the passive voice.
Examples:
2a) The baseball hit him in the chest.
This sentence might be appropriate as an answer to the question "What just happened?"
2b) He was hit in the chest by the baseball.
This sentence might be appropriate as an answer to the question "Why is he all bandaged up?"
2c) He was hit in the chest.
This sentence might also be appropriate as an answer to the question "Why is he all bandaged up?" Here, the agent is unexpressed, or possibly unknown. This sentence is all about what happened to the fellow who got hit.
3a) Andrew Morley wrote this book.
This sentence is about Andrew Morley and what he did.
3b) This book was written by Andrew Morley.
This sentence is about the book itself.
So don't be afraid to use the passive voice, but save it for cases when you want to focus on the outcome or the person or thing affected by the action of the verb. It's easy to overuse the passive; I suggest you go over your writing and substitute active voice wherever appropriate. If in doubt, use active voice.