M.F.
Most cats don't go into heat untill 9 months old (sometimes later) As it is so hard to tell with some cats most vets recommend spaying at 4-5 months old. Call your vet ASAP because it could take a 2-3 weeks to get an appointment thats convenient for you. She will need two appointments. first for a physical and vaccinations, then the day of the operation.
Good luck and thanks for being a reaponsible pet owner and helping to controll pet populations.