You want to take part in the tournament. First thing to decide is whether you will play for real money or for play money. These are totally different - only the cards and the rules are the same. Play-money tournaments are made for fun and for initial training, not for professional playing. Don't expect you will meet Chris Moneymaker here. Players usually don't count their play money because they can get more any time, and spend them unlimitedly. In real-money tournaments, all is much more serious. The players here value their money, act much more cautiously, and don't blow off their money just for the hell of it. Here serious men and women meet - to have fun and to communicate, to earn some money or to make living. Something between is freeroll tournament - you pay nothing to take part in real-money tournament and fight for real-money prize. You lose nothing even if you are the last in the list.
Next classification is sit-and-go and scheduled tournaments. The difference here is in time of start. Scheduled tournaments have a schedule of every event - they last for a certain period, start and end at a particular moment. Thus, you have to sign up to participate in advance. Sit-and-go tournaments start when the required number of players has signed in. It is a perfect play to practice your theoretical skills in poker - because each time you take part in this type, you participate in final hand.
The third difference is single-table and multi-table tournaments. The goal of the first type is to accumulate all chips on the table - if you manage to do this, you win. On the contrary, if you give away all chips, you lose and end your participation in the SnG. Multi-table tournaments enable playing of many players - as soon as one is eliminated from the table, others who desire to play immediately realize their wish.
Thus, choose what you prefer more and enjoy poker as many of others do. Good luck!