Propain asked me to teach him how not to suck at this game. As I'm not THAT cruel I promised to link him to some useful articles on how to start with this brilliant game.
This article basically sums up everything you need to know to start:
How to stay alive longer than one minute. Really, just start as a soldier and spam, follow your teammates and try to stick to a medic. Over time you'll recognise the map (when Badlands was new it took me a week before I realised that the map was mirrored), and where the ammo and healthpacks are. Look at what other players do and try to copy that (eventually). Playing medic taught me a lot, because you can see what better players do from a bit of a distance. Unless you know how to play the class it's not really going to be rewarding to play medic on a public though.
So, play soldier and spam.