The toilet seal is called a "wax ring" and can be replaced fairly easily. The hardest part is moving the toilet. You will have to lift the toilet to remove and replace the ring. First, shut the water supply to the toilet off.
Flush the toilet and hold down the handle. This will allow the tank to (mostly) empty. Soak up any remaining water with old rags. Disconnect the water supply line from the shut-off valve. Remove the covers for the two bolts at the base of the toilet which will allow you access to the nuts that secures the toilet to the bolts. Carefully loosen and remove these nuts. Rock the toilet to loosen the seal and lift it up off the bolts. (Place your hands inside the toilet bowl rim and lift.) Then, carefully, turn the toilet on its side. (Be aware of the tank and the fact that it is bolted to the bowl.) Remember to lift from the bowl to prevent cracking the tank or the bowl.
Temporarily stuff a large, old rag in the floor drain to keep sewer gas from entering your home.
PUT ON RUBBER GLOVES! (If you haven't already!)Using a plastic scrapper, you'll need to clean off the old ring that is located on the underside of the base. Press the new wax ring around the "horn" of the underside of the toilet (where you scraped off the old one). Pack some plumber's putty around the floor bolts to help them stand-up straight. Next, rock the toilet slightly and then sit on it. Yes, sit on it. You want to make sure the ring gets seated properly.
Gently tighten the nuts on the base of the bowl. Do not over tighten the nuts! You could crack the bowl. Replace the cover. Reconnect the supply line and turn the water on.