It depends on what you look for in a poker site. Definitely the best is Pokerstars, for its number of online players, the number of tournaments and variety of types of games. But if you are new and learning, you may want to try sites with less traffic (number of players), for example Intertops. Their interface is not the best but they have cheap tournaments with not many players. A little better is Americas Card Room, this site has free tournaments every 2 hours, in addition to freerolls from many sites similar to Cardschat. 888poker has a better interface and greater traffic, many interesting tournaments and without PS traffic.
All sites allow you to create your account and play without depositing money, so I think it would be best if you tried each one and decided for yourself.