The particulars of rules and character creation are really dependent on the game you play, as well as the group to some degree. There's countless systems out there covering just about every genre you can imagine, from generic fantasy to cyberpunk to Mormon western (really). There's a few families of systems that you can fairly easily transfer skills between, such as d20 (DnD, Pathfinder, D20 Modern), World of Darkness & Exalted, *World (Apocalypse World, Dungeon World, etc.), and Warhammer (Dark Heresy, Rogue Trader, etc.). Besides the rules of the games themselves, it's a good idea to get comfortable with the idea of playing your character, and get a basic grasp on gaming etiquette. Most of that is common sense: be punctual, be courteous out of character, don't be a disruptive member of the group, and so on. On the less obvious end of things: try and bring something to the table. The game isn't just for you, it's for a collaborative effort, and everybody plays their part in making it fun for everyone. Finally, the rules of a given system are never absolute. The GM may alter, bend or break them at times, and this is nothing particularly unusual. Reading the rules in advance of playing a game is still something you should do, but don't be surprised if things don't end up working exactly as they're written in the book.
