Even though Team Fortress 2 has been out for awhile, it is one of the most innovative team first person shooter game out there. Generally most fps games, the game character is generally the same with only the weapons different. What makes team fortress 2 stand out is the fact that the classes match the weapons and define them. This game also has something for everyone whether they are a beginner or advanced from playing other FPS games.
There is a wiki that explains the detail of each class and maps at
http://tf2wiki.net Since with the updates, there are constant changes in some of the attributes of the weapons for each class. For those who never played a FPS game, I would recommend starting off with pyro and medic class. To play these classes all you need is to run around and stay alive. The medic class is also a good starting point for learning how other people play different classes since you are constantly watching them. Pyros are a good class against spies who tend to target people who stand around.
For those who have a Quake background, classes like soldier, demoman and heavy are similar to weapons in Quake. Those who were into Counter-strike, I would recommend scout, sniper and spy. These classes are similar to the weapons used such as the AWP, shotgun and the knife. I would say that the two classes hardest to learn to play is the spy and engineer. The engineer requires the knowledge of the maps, but a lot of the places on the map have ammo nearby sometimes to indicate where to set up sentry gun generally. Yet this does not hold true for all possible areas, it's good to have ammo near by to build. The spy takes timing and patience, but very good spy could dominate the game.
With all FPS regardless of how well it is designed, finding a good server with regular players is the main factor of the online gaming experience. It's hard to get addicted to a game if no one plays. If you are a medic knowing who the good players are helps improves your score since you don't waste ubers on crappy players. Since Team Fortress 2 focuses on team work, balancing the classes helps prevent weakness in the team dynamic.