The world has extremely many online poker rooms, and depending on where you live, you can play at most of them. Some are for US citizens only, others are not allowed for American customers. E.g. Full Tilt Poker is not allowed in every country in Europe, unless you pay some sort of tax, assuming that you want the money you win withdrawn from the poker room.
Americans can't play at any poker rooms they want neither. Some years ago PartyPoker lost a lot of customers, due to a law in the US that forbid americans to play gambling related games at foreign suppliers, such as PKR, PartyPoker and LuckyAcePoker.
One poker room though, is available to all, and legal for all to withdraw money from, and it is PokerStars. PokerStars is by far the most crowded online poker room, and that's definitely not a disadvantage for neither the players nor the poker room itself. There is at any time a free chair at a table with any player's favorite game, and at any different stake that the player wants to play.
What about the number of different games? Well, when a specified type of game is becoming popular at one poker room, it is likely to move across all poker rooms, so today most poker games are available at any of the online poker rooms. The most common poker games today is Texas Hold'em, Omaha, Omaha Hi/Lo (8 or better), Stud, Razz, Five-card draw, Deuce to Seven (Triple Draw or Single Draw) and Badugi. Also the mix-type games such as HORSE, HOSE, Mixed Hold'em and Mixed Omaha Hi/Lo are all offered at PokerStars and all the other poker rooms as well.
Also, online poker rooms need to do marketing to attract new players. The way they do in the online poker business, is to sign a sponsorship with professional poker players and by that binding them to play at the poker room. That's why you see people playing poker on television, that wears caps, t-shirts or other clothing. Phil Ivey, Gus Hansen and Chris Ferguson is sponsored by Full Tilt Poker and they are all bound to play online at Full Tilt Poker as well as doing other marketing stunts. At PokerStars they have Daniel Negreanu, Chris Moneymaker and Viktor "Isildur1" Blom to attract customers, but they also are the headline sponsor at the event EPT (European Poker Tour).
When you want to play poker, you can always find yourself a comfortable seat at PokerStars. It is the most popular poker room, and whether your are a pro or a newbie there is a table that fits you. If you don't want to play with real money, you can just choose to play using fun money or playmoney and with them practice until you feel safe about playing at the real money tables.
If you have a friend that already plays at PokerStars, and you don't have any money on your own account, you can just ask him or her to borrow you some. PokerStars has a feature that allows players to transfer money between each other, and that can be used if you are running low on your own poker account. Just remember that if you want to maintain your friendship, it's recommended to pay back the money you borrow, and you can of course use the same money transfer system to transfer back the money.
If you want a little money boost, you can choose to write a PokerStars Bonus Code when you sign up. If you do this, you can get up to $600 extra on your first deposit - it's a 100% bonus, so PokerStars gives you the same amount as you deposit, so go find a PokerStars Bonus Code, and get extra money, and then play poker to win the millions.