Good idea. Well for starters, I would recommend heavy, pyro, or soldier class. those classes seem to be relevantly easy for new players. I also recommend jumping a lot. Keep moving, jump around to avoid enemy fire.
I also HIGHLY recommend leaving tf2 running overnight for a night. You will wake up with a bunch of new weapons which you will have in your disposal. the stock weapons are good, but I like to mix things up. I only use 3 stock weapons out of the entire lineup. 1 good night on a idling server will get you some great loot.
When I have time I will go more into details on class strengths and weaknesses.