You need a leveling compound. This will be a cement based product that is either trowelable, or self leveling when you pour it. Don't use anything like drywall compound as it will fail eventually. You might find a decent online tutorial for prepping concrete subfloors. Really pretty easy as long as you keep in mind that you are really pouring concrete on a tiny scale. You can sand, grind and fix your work if you need to, but it's a lot easier to get it smooth in the first place. Good luck.
