Cut it, then fertilze it. Don't cut it too short, just regular hight for the current season, then proceed.

Before we proceed... What type of grass?
Asuming you have bermuda then that should be fertilized every month this summer with nitrogen only fertilizer. The more you feed it the more it will grow and the more it will grow, the more often you will need to mow it to prevent scalping. This also depends on your mowing height. Cut to the lowest setting on your mower. With a rotary mower this is generally 1.5" and with that high of a cut, once a week should be sufficient. If mow lower than that then you would wnat to mow more than once a week to prevent scalping.