1. You don't need to. There are prints, and certain bases/adoptables will need you to pay money (or their form of currency on there) but you don't need to spend money to make an account or anything.
2. Not too much, I think some may be optional as well.
3. What do you mean by practice a little? You can practice in other programs on drawing and such.
4. No, you don't have to have a super good drawer. /look at my art heheh/ Deviantart is a place for any art, no matter what experience level.