Probably best to find some friends to play it with, or at least some friends who are open to the idea. If not, if you go to your local nerd-dens (trading card shops and comic book stores are your best bet) you can usually find something about DnD. If they sell unusually sided dice, more likely than not the clerk will know some people. If that's a bit out of your comfort zone, there are a number of websites for playing DnD, such as MythWeavers.
4.0 is the most recent. 3.5 is open source, and can be found online for free (I think. I'm pretty sure its open source, and doesn't cost money for the download any more, but I'm not positive). Pathfinders twists some of the rules around, and some people like it, but I've never played it, so I can't really give you advice there. Ultimately, if you join a group of other people, you'll probably be playing whatever they're playing.
At its core, DnD is a game of imagination. One player acts as the storyteller, known as the Dungeon Master. He is the referee for the game. He creates the setting, the events that are happening, and the challenges that a player faces. Everyone else, the players, are the main characters. The DM presents a situation, and the players decide how they want to act.
DM: A dragon appears. It's angry.
Player: I draw my sword and lop its head off. OR Player: I cast a spell to bolster my allies OR Player: I try to talk with the dragon OR Player:I'm not stupid. I hightail it.
The rules are a means of balance. The rules give Players a rough idea of how strong they are, what they are capable of, and what they can achieve. The rules give the DM a means of presenting appropriate challenges, rewarding the players accordingly, and creating the slew of adventures.
A word of caution: It's a very slow game. To get started, you:
1) need a group of people.
2)The rulebook, for whatever you're playing. There are tons and tons of rulebooks which expand the ideas and possibilities of the DnD world, but at its most basic, you need the Players Handbook, for the players, and the Dungeon Master Guide, for the DM. (for DnD anyway, some systems have these two lumped together)
3) some sort of large grid that can be drawn on and erased. This is for moving your character in a battle. They sell special game mats for this, but its not necessary for a beginning player. A whiteboard with a grid drawn in permanent black ink works fine (my first group used a large piece of paper with the grid on it, and a plexiglass sheet on top of it for drawing things).
4) Some sort of figurines. These indicate your characters, and the enemies. You can purchase fantasy characters, but they are quite expensive. Little paper printouts, coins, or legos all work fine. We used chessboard pieces, with the good guys in white (each character had a specific piece), and bad guys in black.
5)Dice. First off, the book will refer to dice as XdY. X is how dice you roll, and Y is how many sides that die has. so 4d10 would be to roll four ten-sided dice. You'll need a 1d4, 1d6, 1d8, 1d10, 1d12, 1d20, and 1d100 (a second ten sided die rolled in conjunction with the first). If you have any sort of cardshop, you can buy a full set for pretty cheap usually. If not, online'll cost you about 6 bucks.
6) Character sheets. The Players Handbook explains how to make these. You can have each player make them beforehand, or just make them when you get there.
7)Patience, flexibility, and a willingness to compromise and have fun. Starting out is tricky, but good luck!
EDIT: If you're joining a group instead of starting one, they'll have most of these things. You'll just need a character sheet. There's a lot of superstition about using other people's dice, but you don't have to buy your own set (it is encouraged though, since the dice alone aren't too pricy)
