What edition are you playing? 4th is quite accessable, 3 and 3.5 less so. The best way to -actually- learn the mechanics of the game is bluntly, to RTFM. Get the players handbook and read over it. But I can give you a quick bulletpoint guide on 'what you do in D&D right here'
Make a character concept - a human rogue, or a eladrin warlord. Flesh them out a bit, give them a reason for being the person they are.
Stats! Give them their statistics, reflecting the kind of person they are.
High Adventure! The Storyteller (I know it's called gamemaster or something in D&D, but I'm more world of darkness oriented) will give you plot hooks. Follow them for hijinks and other anachronistic phrases! Indubitably!
Explore Dungeons! Do not explore Dragons. Dragons are notoriously small on the inside, and the rewards are similarly tiny. Unless you really need a partially digested dwarf skeleton, go elsewhere.
Kill your foes! If they're orcs, if they look at you funny, if you look at them funny, GET FIGHTING. Then take their stuff.
Level up! Hooray!
and that's D&D.