Chapter one and introducing the main character?

It depends on your style. Just please don't do "Hi, I'm Sally, and I had a heard life." I hate it when the main character acts like he or she is talking to the audience directly. Then again, that's just me. If that's the style that you prefer, go for it. When you rad books, what do you like better?

There is no right answer to this. But readers' tolerance for backstory and infodumping is very low nowadays, so unless you're a better writer than about 99% of the writers who've been published recently, I'd recommend weaving the character's history in over the course of the book.

It's best to let the reader know who or what the story is about early on. Otherwise, the reader won't know what they're reading about.

I always find it interesting when you learn the characters backstory throughout the story