Does Full Tilt, or Stars let you play against CPU? I haven't really played any serious poker before... just wanna practice
If you are going to play online, you might as well play against real people. You'll learn more. Most online sites (Pokerstars, Full Tilt) let you play with fake money. You can also find poker as a facebook/myspace app, but you won't learn as much, the people there play terribly.
Neither Full Tilt Poker nor Poker Stars offers play against a computer opponent. It would be somewhat silly to do so. They would have to write programs to play against you and running those programs would take up a lot of resources (they would have to have the ratio of bots to players be like 7 to 1). This would reduce the number of resources they have for real money play.
Both sites provide play chip games, where you play for a free fake currency. Of course, the people who play at this level are much more reckless than people who play for real money, so it's not an exact simulation of a real money game. In reality though, you can play at a micro-limit game (1c / 2c) and the players are almost as reckless.
If you go to websites fulltiltpoker.net or pokerstars.net you can play against real players with free chips. I am not aware of any sites that allow you play against bots. The .com version of these sites are for real money.
.net = free
.com = real money