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 affect 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.
In Fortress, there are two useful mechanisms to compensate for overly aggressive or downright rude behaviour: silencing and voting.
If there are players who are treating you badly or are just mouthing off too much, you can silence their chat. This enables you to concentrate on your gaming and interacting only with those players who seem to be helpful (or amusing) to you. Of course, you might also miss some important information regarding teamplay, but if another player points it out to you, you have an opportunity to explain your actions. If you silence everyone, of course, you might end up being kicked.
And sometimes it is necessary to vote off another player. Be careful with this option. Sometimes a player (and it may be you one day) is nominated by someone in the heat of frustration, in which case you should reject the nomination. However, if a player is repeatedly team-killing or generating too much negative vibe, then make the decision to approve the nomination with your vote. You press the left and right arrow keys to change your vote. Escape registers the vote.