It sounds like you have a clog somewhere in your sewer line. To clear a clog you have to be smarter than the toilet. First, get a mirror and use it to look deep into your toilet's drain. If you see nothing the next step is to remove the toilet and see if anything is stuck in the lower portion of its drain. If the toilet is A-OK then the next step is to use a "closet auger" (a short flexible coil spring cable that is used to dislodge debris in a sewer line). It's usually pretty safe for a DIYer to attempt to dislodge debris with a closet auger. A closet auger is made for short cleaning runs and doesn't have the potential to damage a sewer line like its full-size big brother. The sales clerk at the hardware store can explain how to use the smaller device. Once the sewer line has been cleaned the toilet will have to be reinstalled. Don't forget to use a new wax wring to create a watertight seal between the toilet and the sewer line ? this is one place you definitely don't want a leak -- stinky. The old wax ring will most certainly leak. A closet auger often solves the problem eliminating the need to call a plumber?