There are some servers which provide a grid where you are playing in a team. Playing can be just like playing alone, with a little selective targetting thrown in. However, in Fortress, it is essential that work with your teammates at the outset.
Check out the various pages which detail how to win respect among your teammates.
Learn a little about grinding before entering a teamgame such as Fortress, because you need to grind your teammates trails fast, normally the centre player:
Teamplay can be intimidating for the first-time player. What you do can severely effect a team's performance and if you have just entered the match at a critical point, the temperatures and emotions of your teammates can run very high. The best way out of this is to do what they ask. Listen to the patient ones. And if you can't hack it, leave, practice a little elsewhere, and return to show them what you are made of.
More detailed discussion can be found Etiquette and Protocol.