My friends have been bugging me to play for ages. I finally caved and accepted a key, and now I'm hooked. Unfortunately, the guys who got me into it don't want to correct my newbie mistakes every few seconds, so I don't have anyone to play with -or learn from.
I've played about ten co-op vs. bot matches, but I mostly play RPGs, so I have no idea how to get good enough to play against humans without annoying everyone!
Any help whatsoever welcome. =]

That's why we're here! Glad to have you join the community. Hop onto TS3 (voice.dotanoobs.com) and introduce yourself to a busy channel. Make sure to ask for advice or help!!

Hey there! A good number of us were in the same position a while back, so this is the right place to be. I can play a few co-op bot matches with you if you want some additional pointers. Otherwise, jump on TS3 and let people know that you are new and if we can get you together with some other fairly new people that should get you some good games in. We also run IHs on Sundays, so be sure to check those out. Cheers!

Co-op matches are great for getting a feel for the game and understanding what's going on, but the honest-to-god truth is sometimes jumping into your first ever few Match Making games with other people who have hardly played at all can be easier than Valve's AI

You just have to keep on playing until you happen to be the one dominating your lan and you'll love it, and you'll never stop trying to do it again!

Bot games are good to get your feet wet, I would suggest going into match making with a group from DN for your first time.

http://www.purgegamers.com/welcome-to-dota-you-suck

This is a great intro that I suggest you at least skim over before going into your first real game. Don't be turned off by the title its pretty tongue in cheek and a great resource for new players.