Welcome aboard,
1. A 20 long to start is fine, but I would go as large as you can afford to start.
2. Bowl for water, substrate, and of course Garters.
3. Regular tap water should be fine, but I like to let it sit for a couple days before I use it. I would not suggest using river water since you are not really sure what is in it.
4. Every second week should be fine.
5. As they grow in size, you can move to pinkies. NO crickets.
6. I have never had to force feed a garter, so I would not expect you would have to. Just make sure they are eating before you leave the store, and you should be fine.
7. That is a personal question, but when you figure it out, please let us know.
Make sure to check out the care sheet on garters. Here is the link.
http://www.thamnophis.com/caresheets...ake_Care_Sheet
Garter make great pets, and are not just for the beginner.
Jason