the best thing you can do is play some Newbie Mixes to get an idea of the basics of competitive tf2
you mentioned wanting to learn to rocket jump. here's a good video about the basics of rocket jumping. here's another good video about the idea of learning to rocket jump. check the description box for more videos on basic advanced rocket jumps.
if you're interested in content designed to help you improve your skills in a competitive setting, you can check out Marxist's mentoring videos and the MyGamingEdge tips and tricks series
good luck and have fun :)