When you get the warning, you should see a little box appear that tells you which cell has the circular reference. Basically it means that the from cell A is reading from Cell B, but Cell B is reading from Cell A, so the formula is going round and round
e.g.
Cell A1 formula = B1+C1
Cell B1 formula = A1+C1
Once you know what cell has the problem then you can change the formula.