I've been playing since early november (and played HoN for like a year before that) so I like to think I'm pretty good at this. My tip for this game is not to jump right into matchmaking but to make a private bot game to familiarize yourself with the different heroes. The bots are surprisingly tough, and better than human players in some aspects. It also helps that they won't yell at you if you do badly .
Feel free to add me on steam, I'm always looking for more people to play and I don't mind showing newbies the ropes. http://steamcommunity.com/id/princesstensei
Oh, if you DO go into matchmaking, be sure to tell people on your team that you're basically completely new to the game (even if you think you're a pro at league of legends), I find that it affects peoples attitudes towards you a lot (in a positive way), and it might even bring a few people out of the woodwork willing to help you.
Also, DON'T worry about denying. Denying is something that only has a very slight impact on the game (similar to having runes that boost your xp gain in LoL), and on lower levels barely anyone will be doing it anyway. It's worth learning when you start getting better because the earlygame is really all about racking up the slight advantages that will put you ahead, but the main priority for a new player is to not die.