Click on the "+" to the right of each roster position to select a player for that roster spot and collect their Tier Points. Your roster is valid when you have selected a player for each position and you have collected at least 50 Tier Points.
First register, and then create or join a league.
To create a league, just pick a league name and password when prompted during the registration process and then send it to your buddies. Or, if one of your friends has already created a Fantasy 100 league, just select "Join a League" after you've registered your team and type in the league name and password provided by your buddy.