You'll need to remove the toilet to level the surface.

To remove the toilet:
1. Turn off the water supply
2. Flush the toilet to remove as much water from the tank as possible
3. Unscrew the water supply from the toilet tank (make sure you have a bucket underneath to catch the excess water)
4. Unscrew the floor bolts and store them safely
5. Lift the toilet carefully with your legs and place on a rag or piece of cardboard, making sure you don't tip it too much so water doesn't leak out the bottom
6. Make sure the surface is level and the toilet bolts are securely attached to the floor. If not, you may need a new floor plate and bolt package. To level the surface, you can place plastic shims under the front or back after the wax ring has been replaced.
7. Replace the wax ring (this is important to do every time you remove the toilet to ensure a good seal)
8. Replace the toilet, threading the bolts through the holes on the toilet, tighten the bolts 1/4 turn past hand tight, re-attach the water supply and turn it on.

Shim it! (look for cedar shims at your local hardware store)

(Chris' answer is far better, though more involved)

If it there is no water leaking then you can tighten the closet bolts, those are the two bolts that fasten the toilet to the floor. If the toilet is level after you tighten the closet bolts then that should be all it was, however if the toilet is leaning or becomes loose a shot time after you have tightened it then it may be your sub floor is becoming weak or rotted (if it is on wood) under your flooring. If that is the case then get a professional to have a closer look. It could also be the issue that the tiles are not level, in that case if there is no water leaking, then yes, shims will be the best solution. Hopefully it is not rocking a lot, did the idiot in question apply silicone to the base of the toilet? if not you will want to do that after you shim it tight. The silicone will help make it look better if it is uneven. Tile is a tricky business and needs careful attention when being installed. If you have extra tile left it may be worth the time and effort to replace the tiles under the toilet to insure they are level. Nothing worse than a bad tile job. I hired a guy to my house and he wrecked my floors to the point they were unusable, so I fired him, ripped it all out and did it myself, now you can slide a quarter across the floor from end to end. But I did not get any money back from the guy so I was out 2200 dollars,, a major drag.