It's a bitch to spike if the sides arent super short/shaved. I would not recommend spiking it and then cutting it because you will probably end up spiking hair into the mohawk that doesn't belong there. Then the hawk will be all uneven and crooked.
What I would do would be to go to a pro and get it cut..atleast the first time. He/She will make sure the hawk is even and straight :D
Good luck with your mohawk though!!!!