http://www.darkshire.net/~jhkim/rpg/whatis/tabletop.html makes for a decent introduction to the basic principles, although it doesn't mention the use of dice to determine outcomes of things which are not certain (e.g. given a plain, simple, unlocked box any attempt to open it is likely to succeed, so we can assume the attempt does. OTOH if two equally matched characters engage in combat, then the outcome is rather less certain and dice are used to decide (in combination with the skills and abilities of the character).
You need a minimum of two players, a games master and someone to take control of a player character. I find 3 player characters is a sensible minimum and 6 a maximum. You can go lower (which I find less fun) and higher (which makes for lots of waiting around while the actions of the other five PCs are resoved).