If flushing the toilet causes sewage to back up in your bathtub or another fixture, it usually means there is a clog in the main drainpipe—the stack.
Though you can attempt to snake this out, you're usually better off calling a plumber.
If your toilet is clogged but not backing up elsewhere, don't try to flush it or it may overflow. Instead:
1) Remove the lid from the tank and raise the flapper valve to let a little water pass through so you can see whether the toilet is indeed clogged.
2) If it is, first try plunging with a bell-shaped plunger.
3) If this doesn't do the job, snake out the toilet with a special closet auger.
4) If that doesn't work, call a plumber or drain clearing service.
