Great choice!!! "paper diapers" are chock full of chemicals, and I only use them when we travel. I can't stand the smell of them!
For the diapers: Get about 3 dozen prefolds.
For the covers: About 6. I like the bummis super whisper wraps.
For fasteners: Get Snappies--about 3-4
For a pail: get two liners (so one can be in the wash, and the other in use) and any large, lidded trashcan. I got mine from "organized living" You do not need to put any water in it. Just having the lid on will control odor.
AND--get a cheap spatula to help you scrape poop into the toilet.
If your son is a big pee-er, you'll need some liners, too. Same as the number of diapers.
And when he gets older, you'll need some "toddler sized" diapers to "double bag" him at night.
For detergent: any locally available no dye-no perfume will do. Wash 1 wash (no rinse) with soap and some Oxy Clean on cold/cold. Wash a 2nd time (wash and rinse) with a little soap on hot/cold. Then dry. If you're good, you'll pull out the covers before putting them in the dryer, but I only do this 1/2 the time....
I've bought all my diapering stuff from GreenMountainDiapers.com, and the owner gives GREAT advice, even if it means she refers me to another vendor!!!!!!!
Congrats on making such a wonderful choice for you, your child and the environment!