I just got a Dota 2 invite on Steam.

I have no idea what the fuck I'm doing.
I get the basic idea of what's going on: there's three lanes, creeps attack towers, heroes get exp/loot killing creeps and other heroes, and eventually you push forward and make your way to the enemy base and destroy it.
But there's just so much more crap going on, heroes classified into roles without any explanation of what they all mean, item shops, like a bazillion different items that all do different stuff. I figured if Valve were going to get this MOBA stuff pushed to a wide audience by pulling a TF2 with it they'd do a better job of explaining what to do.
What the heck should I actually be doing? Killing attacking creeps? Roaming the jungle for the creeps just sitting there? Killing other heroes? That seems pretty hard, they just run off or behind a tower.
All I did was set up a private lobby and try against a bunch of easy bots, but I don't think I really got anywhere. I think I had a K/D of like 1/24.