If you aren't good enough to get picked for pugs, play lobbies. They're shitty in so many ways, but they are useful to suffer through and learn from if you're fairly new. Play a bunch of different classes once you get a decent feel for one of them, and if you run into someone nice who seems to know what they're doing, you can ask them questions. That's how I met my first mentor.
Also, I haven't really tried them, but it could be worth trying this map pack (tr_medic) just for the mechanical stuff:
http://gamebanana.com/maps/186499