I've found most GMs are happy to help out new players by teaching them the game, so personally I would suggest just diving in and explaining that you are new and want to learn. You'll find most experienced players are more than willing to accomodate you. There are a few points worth covering though, that will make getting started a lot easier. I'll be assuming everyone here has some knowledge of what roleplay is.
The BasicsThe purpose, of DnD, or any other RPG system at it's most fundamental, is two things. The first is to establish a convenient, consistent setting to play in. A good chunk of most rulebooks is "fluff", background information that enriches the world the RPG is based on. The second is to add an element of chance to your roleplay, so that neither you nor the Game/Dungeon Master knows exactly what will happen.
Sadly, there are some disadvantages to using a system(like DnD). It's easy for players to get caught up on the mechanics of the game, trying to twist them to their advantage, becoming less about creating a story, and more about winning the game. The reason I mention this is that I feel it tends to dishearten a lot of players, and put them off when their first attempt at DnD turns into this. A good DnD game can be every bit as deep and compelling as a good freeform(Systemless) game is.
The most important person is making this happen is the GM(Game Master/Mistress, sometimes DM, or Dungeon Master/Mistress instead). The GM typically does not play a character, but instead plans the story, and chooses when and how to enforce the rules. The GM's goal is not to defeat the players, or even to guide them through his/her story, but to build the story up around the players, giving them the freedom to act as they see fit. A good GM should serve the players, but they also need to keep them in check. Good GMs are really hard to come by, so make sure you show them respect.
Getting started with DnDDnD has had several different editions over the years, but the different editions are actually almost totally different games! You'll want to know which edition you're playing before you start. Fortunately, the GM usually makes it very clear. Here's the two that you probably want to know though.
DnD 5 - The latest(current) Edition, DnD 5 is considerably simpler, and less mechanically heavy than previous editions. It's quite easy for new players to get the hang of, and experiencing a huge resurgence in popularity.
Pathfinder - Ok, so it's not technically DnD. However, Pathfinder is just as popular, and based very closely off an old edition of Dungeons and Dragons called DnD 3.5. The setting is basically identical to DnD, so if you play one, you'll have a pretty good idea what to expect from the other. The key difference between pathfinder and DnD is that Pathfinder is much more complex, and has a massive amount of material written for it. Finally, while having the books can be helpful, everything a player needs to play is available online, for free.
Legally.
Once you've decided what you want to play, the single biggest thing you can do to help yourself, is to find a rulebook or starter guide, and just skim it, to begin familiarizing yourself with it. There's no real need to slog though the number crunching, just reading whatever takes your fancy will still help a lot, whether it be short stories, or information on races, or play examples. When it comes time to create your first character, it will pay off.
Anyway, hopefully you find this info helpful, and I'm not just waffling...
DnD can seem daunting to begin with, but it only gets easier as you go, and the payoff is well worth it. If there's only one piece of advice I can give you to take away, it's that, just like any other roleplay, it's the people that make the game, not the rules. Get a good group of people, and DnD will be
great. Get a bad group, and no amount of rules will fix it. Good luck!
