Scout: Really fast and loads of fun, except you NEED to really know the maps and the classes to not die and be effective
Soldier: Moderate difficulty, rockets are very effective but need to learn how to manage only having 4 at a time, so takes some getting used to. But if done right VERY destructive
Demoman: Very easy for camping doorways, corners and control points. Hard to get used to for 1 vs 1
Pyro: Easier to learn, Hard to master. Everyone always jokes about pyro being m1 and w (ie, fire and walk), but if you learn the maps and how to surprise people you can be DEVASTATING. Also, engies always appreciate spy checks with the flames.
Heavy: Pretty easy to learn, but once again hard to master. You are a target for spies and snipers. Hard to get used to the slow speed and the winding up of the gun
Medic: Once again, easy to learn, but hard to master. Tips: Don't just stick with 1 person, heal everyone. With that being said, find a good medic buddy but keep in mind other people need help too.
Spy: Hard to master. Easy to get 1 backstab, but to escape, live and strike again is harder. But quite fun
Sniper: EVERYONE always goes sniper, and it pisses people off, because then the team loses. That being said, having no snipers is also sometimes bad.
Engie: Once you know the maps, you will learn the standard engie locations and perhaps come up with a few of your own (I know I have, some which have caught on). Tips: Upgrade tele during startup if you are attacking, if defending build tele entrance, then go and buidl the rest at a particular location, upgrading sentry first.
Should be pretty straight forward from that. If you need alot more tips just visit tf2wiki.net . They have tips for every map and class. VERY HELPFUL