Team Fortress 2 - General Guide
Newb No More
Team Fortress 2 is a well balanced game with highly defined class roles. Each class has a set of strengths and weaknesses that fit into a specific set of tasks. If too many players are using a particular class on the team, it becomes less effective, encouraging players to build teams with class diversity. You'll rarely find a team composed of all snipers like you will in some other games, and if you do, they'll probably be rolled over in a matter of minutes.
Few people are able to play every class very well. It helps instead to focus on between one and three classes to get better at. Play as the classes that you enjoy rather than pushing yourself to improve as a class that bores you. It's good to have a good sense of how to play more than one, however, because there will often come a time when your favorite class is ineffective or your team will need a certain class - such as when you love pyros, but the opposing team has five of them, or if your team doesn't have a medic.
As a team, you want to move in clusters more often than not. Medics should have enough time to build their ubercharge so that they can lead a charge forward with the entire team simultaneously. Ineffective teams act as if they were composed of individual players playing alongside each other who are all interested in maximizing their score. More successful ones will leverage the specialties of each class to ring in consistent victory.
Why should you care about your team? It's much more fun to win than it is to lose. It's more rewarding to bring a team together for a common purpose. It helps to know what every class can and cannot do. If you find yourself struggling, step back a moment and think about the other team's composition and strategy. Focus on how you spend your time alive. Are you dying too early every time? That indicates that you are playing too recklessly. Try to find another angle of attack if you keep getting shut down.
Are they playing too many of a particular class? For example, if there are too many pyros on an opposing team, switching to a heavy will likely net you a lot of kills. If there are an unusual number of heavies, switch to the sniper.
The following guide series details how to achieve high levels of individual performance using every class in TF2.