Try all of the maps. Try all of the classes. Have fun.
Three most common game modes.
1. Capture points; like dod taking flags but often it's a linear process. e.g. Must own a and b before you can get to c etc.
2. Capture the intelligence (flag) return it to your base.
3. Payload; push a bomb down a railroad track. Like jungle ball in gym class except one side is all offense and one side is defence.
Classes:
Scout; very fast, light health.
Spy; stealthy can disguise and turn invisable and sap engineers structures. Lethal backstab.
Engineer; can build dispensers (ammo health metal), sentry guns, teleporter pairs.
Heavy; Mobile machine gun, inaccurate at long range excellent health, slow.
Sniper; Just like DoD, backup weapon machine pistol.
Demo; pipe bombs (timed grenades) stickies (remote activated limpet mines)
Soldier; Rocket and shotgun
Pyro; Flame thrower, good for finding disguised and cloaked spies, effective close range.
Medic; Heals players and can make themselves and the people they are healing invulnerable or give the person they are healing critical hits for 8 seconds if they have healed enough points. Fast but poor health. Slowly regenerates own health.
Engineer and Spy have the most new things for a dod player to learn. You may want to get used to the other classes first.
Check out this site as a reference.