TF2 is a good game, just experiment with the classes and choose the ones you like. You have to work as a team to win the games.
Scout: Fast Class, used to capture points or flags.
Pyro: Close range damage dealer, good for breaking up groups. Can't Burn. Easily find spies.
Soldier: All around class, mostly used for 1v1.
Heavy: Slow but steady class, can break up camps, deadly when paired with a skilled medic.
Demoman: Used for defending a specific area using bombs, or taking down sentries.
Engineer: Primary defence, used to keep hold of an area or advancing your team forward quickly.
Sniper: Long range, quick kills to pick off important targets (Medics) and weaken rushes.
Spy: Sneaky class to hide among the enemy ranks and kill enemies from behind or sap sentries.
Medic: Healing class to keep your team alive. Ubercharges make you + other invincible to break camps.
Each team member must do their job to help another class do theirs. Scouts cant cap the intel with sentries gaurding the place, spies should sap these. Your intel is going to be capped without defence. Engineers and demomen can solve this. Snipers can't snipe and engineers can't build with spies destroying everything, pyros can easily solve this. Medics can't heal with enemy snipers, other snipers, soldiers, or almost any other medium/long range class can solve this.
Plan out when you are going to attack or rush, a medic must plan when he is going to uber, ubering is usually done to destroy a bothersome sentry, if a scout can run in while this is happening, you can easily run through this opening to get to the intel.