From my understanding, you are not only new to D&D but to tabletop role-play as well.
Briefly, a character is composed of attributes, skills, feats and abilities that grow over time to make him or her more competent. This is intuitively called experience. As a character gains experience, he gains levels, which increase his or her prowess in key sections: fighting ability, defense, skills, saves, etc. The character becomes complicated very quickly, as you will have to be able to recall a number of numerals.
I suggest that you look at the wizards.com "How to Play D&D" tutorial. It's informative and easy to register. If you have any questions, you should post them in this thread.
As for the versions, people are stuck in either 3.0, 3.5, or 4.0. The differences between 3.0 and 3.5 are cosmetic, and if you understand one you should be able to understand the other. 4.0 is a far more simplified, video-game-like version of 3.5, but it's only simple if you focus on only your character. If you're a GM, 4.0 can get mind-boggingly bothersome with the amount of information that's given.
Now, my beef with 4.0 is that I prefer 3.5. The reason I prefer it is because 4.0 lacks the level of customization that I enjoy in character building. However, 4.0 does provide an excellent repertoire of abilities which make even a mundane hack-and-slasher a diverse, tactical unit. The happy medium between 3.5 and 4.0 is the Book of Nine Swords, which possesses 3.5's level of customization and 4.0's level of action diversity.