TBH, if you're starting out with $20 don't play MTTs. The variance is crazy and you'll lose your Bankroll before you start. With 20, I'd play $1 Sit & Go's or 0.01/0.02 cash tables. Make sure you take in some of the information you can find here before you take your money to the tables. Even reading through a few basic articles on things like hand selection, position and basic S&G/Cash game strategy will probably help you out a lot. Also, I'd look into Bankroll Management before you start.
Assuming you're non-US, the best site to play on is PokerStars
. They have the most traffic, great ease of use in terms of the lobby and table features and an all round good company. Full Tilt is also popular, although the all round good company thing doesn't apply to them.