It overflows either because the drain is clogged or it is not vented properly. Somethng to try that doesn't cost anything. Get a 5 gallon bucket and fill it 1/2 to 3/4 full of water. Pour it into the...
If the stool is the lowest point of plumbing in the house it could be several different reasons. If stool is not lowest and the lowest drain point is not backing up, then you have a plugged toilet.