Instructions
Download the book (see Resources below). Paizo Publishing offers the book for download at their website or you can buy the book for $32 from most bookstores. The download is free and you can decide if you like the game before you buy it. You must sign up for Paizo Publishing's website to get the free download, but the company does not bombard you with spam or junk.
Gather a group of gamers. Like many role playing games, Pathfinder is best with 4 to 6 players and a game master. Check at the local game store or comic shop to find other gamers. Ask about their game style; just because they love the game doesn't mean that they play the same way. Some gamers love the dungeon crawl and others are more interested in stories with a plot.
Read the book. Get an idea of the big changes that Pathfinder has made over D&D 3.5. For example, fighters in Pathfinder get a bonus to their armor class every few levels and clerics can use their turn undead ability to heal people. In Pathfinder, the turning ability is actually channeling positive energy, which heals living creatures and damages undead.
Decide which, if any, D&D materials will be allowed in the game. Pathfinder is completely compatible with D&D 3.5 and any D&D material not replaced with material in the Pathfinder book can be used without any modification. You need the D&D 3.5 Players Handbook, as the spells for Pathfinder are taken directly from 3.5 and are only reprinted if changes were made.
Role play! One of the biggest complaints about Dungeons & Dragons 4.0 was that it over-simplified the game and took out the role-playing element of the role-playing game. Many critics claim that the new game is little more than a live action video game with miniatures.