What initial velocity is required for a bullet to enter orbit?

Question

I need some help for a school assignment. I'm trying to make a differential equation to solve in Comsol. The part I'm having trouble with is the fact that air resistance and gravity changes based on the bullets vertical position. I have absolutely no idea how to do this and without it the bullet would always be equally attracted to earth. With the only exception being the drag decreasing with the bullets velocity. If you know of any way to do this, it would be greatly appreciated.