Not sure about which version of the classic Dungeons & Dragons role-playing system you should start with?
Pathfinder is based on the Open Game License for version 3.5 of the aforementioned game system and is often referred to as ‘version 3.75’. The Pathfinder system improves upon D&D 3.5 by balancing classes to simplify leveling so that it does make sense to level to 20 using one class, providing better options for skill selections, rebalancing some spells, and many other subtle changes. The best part yet? The rules content is available free online: http://www.d20pfsrd.com. Even though the content is free, the books are fantastic and definitely well worth the investment to pick up.
The Core Rulebook is just that—everything you need to play a basic game except the monsters.
The first few chapters are dedicated to the basics—Getting Started, Races, and Classes. Skills are improved in the Pathfinder system in that you are not punished for taking non-class skills, but you are rewarded for taking a class skill with a bonus for skills that you’ve invested in. Four pages are dedicated simply to listing feats in a table nearly identical to the one you may know from 3.5 and another 30 take up the descriptions of the feats. Basic Equipment, Additional Rules, Combat, and Magic are also outlined in full chapters before 150 pages are taken up for Spells. Prestige Classes, a few chapters dedicated to basic Gamemastering, and Magic Items are also included in the book—effectively combining the Player’s Handbook and Dungeon Master’s Guide from 3.5.
In lieu of a Monster Manual, Pathfinder keeps their creatures in the Bestiary. Pathfinder’s Bestiary volumes (4 currently are available) maintain full page descriptions for each monster with an easy to read stat block, full color illustration, and at least a brief description of the creature’s origins and/or disposition. To reference the prior mention that classes were improved upon, realize that most of the basic 11 classes have at least 10 class variants—if not many more—to add customization to your characters without cross classing or hitting prestige classes at the earliest opportunity. Perhaps you might choose to play a paladin that does not utilize the paladin spell list, but instead focuses on his martial ability with the lay on hands option proving more powerful in absence of the spells? Or maybe you’re looking to play a monk that uses a bow instead of her fists? These options and dozens more are made readily available with Pathfinder.
All of this only covers the basics of the Pathfinder game set-up. Paizo offers fantastic modules for their games that include references to creatures, traps, or spells by which books and page they can be found on. So what are you waiting for?