Easiest: Pyro, Scout
Run, shoot, retreat, repeat. When you're below half health, RUN AWAY and get healed. Especially true when you're on defense. That said, you'll die a lot, which will be very useful. Every time you die, learn from it.
Achievement farming does not mean you're limited to farming in a achievement servers =\. You can still farm in a regular server so your stats don't get borked duuuuuuur.
I would say, try them all out then choose the class you liked the most and do research on it...playing styles as well as what type of class they are and watch a few vids on how you can play with them, these things helped me out alot and showed me alot of new moves that I never knew i could do as spy. Ive got over 200 hours as spy and still dont have all the kinks out yet, but as soon as you find a class you like, stick with it and get to know it inside and out, only then will you be able to see how other classes are played. You can see alot of vids on how to play all classes on Youtube, incase your interested.