Building a Bankroll
If you don't have any money to spare, the freeroll is your best poker friend. A freeroll is a tournament that you can play for free, but win real money. As you might guess, the amount you can win is small, and the number of competitors is large, but if you keep at it, you can win money this way. Freerolls are often offered to new players, and you can earn points towards freerolls with real money play. You may also be able to win a satellite into a freeroll. The catch is that you need a real-money account to participate in most freerolls.
Moving up the Stakes
Eventually, when you win that first free money, you'll want to jump right into a real game. However, a dollar or two won't take you far in most games, so you'll have to start out with micro stakes. Remember that poker can have a great deal of variance, and you want to have a lot of chips to play with, so taking your two dollars to a 25 cent/50 cent game is not recommended. Taking it to a 1 cent/2 cent game is a much better idea for your most likely tiny (at first) bankroll.
Protecting Your Bankroll
You'll want to move up to real stakes when your bankroll is big enough, but you should have at least 500BB for the game you want to play before moving up in stakes. This doesn't mean you'll throw all this money on the table at once, but you'll want to be able to re-buy or withstand a few losing sessions, which are unavoidable as you work towards the best online poker games. Your goal is to avoid going broke before your skill takes effect, so the more money you have to protect yourself against this eventuality, the better off you'll be.