Try an Auger
If a plunger does not work, try a toilet auger (snake). You can get one at most hardware stores for $20 to $50. The trick of a auger is to get the cable right through the throat of the toilet. Put a little tension on the cable and start turning the auger, extending the cable down into the drain pipe. As you push down you will feel the auger drop. Once the auger approaches the clog, give it a little extra tension to punch a hole in the clog, creating a line to the sewer.
A plunger may also be used in conjunction with an auger in order to create enough suction to blast out any material that may still be clinging to the hole that was punched out by the auger.
Note: The toilet and the bathtub normally share a drainage pipe, so if the clog is too impacted for the auger to punch a hole, the whole clog could be shoved so deep that the bathtub becomes clogged as well. Once the clog in the toilet is removed, be sure to check your bathtub drainage as well.