A good guideline is to take 40 max buy-ins for the level you want to play.
For example, at the $0.02/$0.05 tables, the max buy in is $5, and you want 40 buy ins to regularly play the level, in this case $200. This is conservative bankroll management.
Now, you have to start somewhere and if $20 is all you got, it's all you got. Personally, I'd recomment playing the smallest increment of Sit N Go you can find and look to build your roll. If you want to play cash games, play the smallest table you can find and be disciplined to build your bankroll.
Move up to $0.05/$0.10 tables when you have $400.
Yes, it's a grind, but this is how you don't go broke. (I've gone broke several times, and am now working to play this disciplined approach).