Assuming you understand the basic rules and elementary concepts, your first step should be at a play money table. Initially, concern yourself with learning the proper structure of the game, how blinds work, minimum raising amounts and even something as simple as becoming comfortable playing poker on whichever site you choose to play.
Once you get a feel for the games, you need to deposit real money. There are many options for doing so, a discussion of them can be found by clicking here. Poker Curious suggests you deposit a large enough sum to play comfortably whatever game type you choose. For advice on what game to play, please click here. To learn about managing the money you put online, known as your "bankroll", follow this link. With a fundamental understanding of poker and the site, you are ahead of 99% of players first starting out.
Now, the most important thing you can do is resist the temptation to play a limit you are not capable of beating. Sitting at a $50 sit n' go when you only deposited $200 is a surefire recipe to get frustrated and lose all your money in a matter of hours. Stick to $5 SNGs to start. If you want to play a ring game, stay at $10 or smaller.
Let's be honest. Your initial deposit is not likely to last long if you do not take measures to improve your game. There are hundreds of ways to do this. You can join a training site, read books, visit forums such as 2+2 or find somebody to coach you. Poker Curious has advice on what you should do; just browse around for links to articles, instructions on signing up for a training site and a wealth of advice on all things poker!