Before you figure out what to do, you should ask yourself what you want to do.
However, if you have no experience tanking as any class, which I'm assuming you don't, I would recommend staying ret. While you beat on things in heroics, and eventually raids, watch and learn from the tanks. Pay close attention to the mechanics of each encounter. Also, roll on tank gear for off spec and start building a tank set. Once you get the gear to handle tanking heroics*, offer to tank them for your friends and/or guild, as they'll be more forgiving.
Here's a link to the
DPS rotation from the best ret pally site. I leveled prot and when I wanted to learn how to dps this is where I went.
Here's the
talent spec (with glyphs) you should be using.
Here's an
extremely helpful video from Tankspot.com. It'll teach you the most important aspect of being a good tank: situational awareness.
In short, stay ret. Learn from your tanks by watching and asking questions while building a tank set. Same goes for healing. Talk to healers while building a healing set if that's what you want to do.
*535 Defense to become uncrittable in heroics, 540 in raids. 25-27k Health unbuffed (depending on your healer).