This is actually simple, though perhaps laboursome at first:I actually have no idea where I would even start with 3d game making because I feel like there are so many options and I really need to start out with the easiest option
1. Grab some XNA sample that has some 3D camera
2. Create a XZ plane with Y=0 that will temporarily serve as a level terrain - just repeat some texture at first - just to get sth on screen as fast as possible - this is very important, because internally, the gameplay logic will be 2D (e.g. only X and Z position, since ypos=0). Later you can add the terrain, where the ypos will vary.
3. Understand the camera and how it works - make sure you can , at any time, in your main class get the camera loc and orientation and its view matrix
4. Start rendering some cubes above the ground.
5. Promote few cubes into "Enemies" and start moving them along a series of waypoints (e.g. "Patrolling") - create a FiniteStateMachine for that with states like ("Stand", "Still", "Walking","Dead")
That should keep you busy for a while