What do you need to have backyard chickens?

Image by Rachel Tayse
Update on backyard chickens at houndsinthekitchen.com/2010/03/15/update-on-the-girls/

Question by Erin: What do you need to have backyard chickens?
We have a 30×30 backyard and would like to have some chickens. I was wondering how we get started. I would like white eggs, and and was wondering how many chickens I can have and if different breeds can live together. Los Angeles area, so any breeders and where we can get supplies too. Thanks

Can you help? Leave your own answer in the comments!