Hi and welcome to Cardschat! You're asking a great question.
The basic rule is to have between 20-50 buy-ins for a given level. This is based on having 100x the big blind as equal to one buy-in. So for $0.01/$0.02 games, that would be $2.00 for a buy-in.
$20 only gives you 10 buy-ins at this smallest level. Don't play any higher cash until you get to 40-50 buy ins and then you can think about moving up in stakes.
Many other new players have asked this question. You can search in this forum for "bankroll management" for cash games and you'll see lots of good advice. (And some bad advice too
) A good post is this one:
https://www.cardschat.com/forum/cash-games-11/building-micro-stakes-bankroll-177533/
Really important to keep hand histories and post them in our forum so that you can get feedback and improve your game. You won't win, even at the micros, if you don't work on your game regularly.
Best of luck!