I've entered my serial number and it saying it is not correct.

After following sberman's advice, do you still get any error messages? Like "Invalid Serial Number?" If so, then this message generally indicates that the device you have was serviced or even replaced prior to your ownership. If you purchased this device new and have never had it serviced or replaced, then contact Apple Support directly and they can correct this error for you.

Serial numbers may contain a numeric zero, but will not contain a letter "O".