How Much do I Need?

How much do you need to set aside for your bankroll to play poker? Now, no one has actually asked me this question, and I'm not entirely sure that you are asking it either. But if you would ask me, I'd tell you that conventional wisdom would say that it's the wrong question to ask, and recommend that you instead look at how much you have, and ask me what kind of limits you should be playing with that size of a bankroll. And conventional wisdom has a point. How much to put in your bankroll is entirely up to you, but finding a suitable limit to play given a specific amount is fairly standardized; in fact, almost everywhere you turn, you're going to be recommended to have 300 big bets worth of a bankroll for limit games (slightly higher for NL). So if you play $0.50/$1 limit hold'em, you should have a bankroll of $300.
But where conventional wisdom may fail you is in the facts that
The 300BB recommendation is based on standard deviation, and how much a break-even/winning player can be expected to survive on through almost all downswings. You may not be a winning player just yet.
You might not be interested in knowing how much you need to play $3/$6 - you might simply want to know what the smallest possible investment that you can realistically build a real bankroll with (without having to redeposit) is.
So, while conventional wisdom may fail you, I will try not to - even if the opening paragraphs seem intentionally confusing. Here's my answer - ready for it?
$50.
Yes, with $50, you can build your way to fame and fortune in poker. If you can't scrape up $50, you shouldn't play real money games at all at this point. You're better off saving up until you can set aside the full amount, because I think it's important that you get started properly. Depositing $20/month and trying not to lose it might work for you, but if you take two months off from real money games and instead buy a book and study while saving up, I think you will be successful a lot sooner.
What about play-money games?
Ah, there's nothing wrong with play-money games. In fact, I encourage you to try out sites by playing these games. It gives you a feel for the software, and you might even learn a thing or two. It's important to realize, however, that these games, where nothing - besides possibly self-respect - is at stake, do not constitute a suitable learning ground for a poker player. It's okay for getting the very basics down; understanding the rules, the betting structures, what happens when the pot is split, etc. But poker is a game of risk, and you will learn very little if you're not exposing yourself to risk.
However - and this is important - contrary to what many others may tell you, play-money games are not "a crapshoot" where it's just luck that determines who wins. On the contrary! If you're disciplined and skilled, these games are entirely beatable, and an expert player would make an absolute killing. Don't let anyone tell you otherwise. The fact that your opposition will be playing like complete donkeys is not the reason I don't think you'll learn much from playing in these games, however. No, the reason is that I'm afraid you will play like a complete donkey, and not really get a chance to realize it.
When you're depositing real money, and playing for real money, you can't reload for free whenever you decide to go on tilt and bust out. You can't click on a button that will automatically give you more chips; you need to get your credit card out of your wallet. This is important, because doing this reminds you of the fact that you lost money. You lost - and now you have to figure out why. In a play-money game, you can easily fool yourself into thinking that you're probably ahead, because you don't remember how many times you've requested additional play-money chips. If you're just now starting out, you have a lot of harsh lessons to learn about discipline, and the sooner you learn them, the better.
So, to sum up: Play-money games are fine for learning the basics of the structure of the game, but may fool you into thinking you're better than you really are. Come on over to the real-money side, but find a limit where you can afford to play is all I'm saying.
Where do I deposit my $50?
This is really a matter of personal preference. You may want to read on, and look at the chapter on bonuses, but in the end, it comes down to where you feel comfortable playing. I've bounced between a lot of different card rooms, and usually find something I like in most of them, but I can't say what's right for you. If you do start with the $50 I recommend, however, make sure that the cardroom you're about to deposit money to offers games in the $.10/$.20 region of stakes (or even lower). Depositing $50 and finding out that the smallest game available is $.50/$1 is bad news; your deposit may not last long if you sit down at those tables.