If you toilet has been rocking the cause may just be loose screws. If your toilet is rocking, tighten the two screws that you will see on the base of the toilet. There is one on either side. DO NOT OVERTIGHTEN!!! You risk cracking the porcelain if you tighten these screws too much. Now see if the toilet is still rocking. If there's no rock, then you've go it fixed.

If it's still rocking, either the toilet flange is broke (cracked) or the floor is uneven. (This happens a lot with tile floors) If the floor is uneven, you can shim it. If it's not the floor then watch the video on how to fix the flange. It's much easier to get the replacement piece shown in the video. Replacing the entire flange is not an easy task. Also make sure to replace the toilet wax ring, that goes between the base of the toilet and the toilet flange.