Probably the easiest to get started with would be either D&D 3.5 or Pathfinder, because they have both the books/pdf's for reference, and the online SRD's (System Reference Documents. Then there's always the old-school games and retro clones such as Original D&D, Swords & Wizardry or Corruption. These are a lot less rules heavy, with simpler mechanics and less overall character options, making them quite suitable for initially learning to play.
These are mostly fantasy setting games, but many GM's do implement more modernized settings to play in as well.
Do you have an idea of a setting you want to play in? Fantasy? Horror? Steampunk? Cyberpunk? Supernatural modern? etc etc