How to implement top down shooter 2d controls in a 3d environment?

Hi, im trying to make a top down shooter where you can control your character's movement with the wasd keys and and use the mouse to control the direction your character is facing, using left click to fire. However, my game is in a 3d environment and the terrain the player is standing on isnt always flat so most standard tutorials for top down shooter controls dont work.

How could i go about coding something that accounts for the fact that the character isnt standing on a flat 2d plane?