There is no better way to learn about 3D printing and robots than building your own 3D printed robots. The Ardumower is a robotic lawn mower you can build to mow your lawn conveniently. It is driven by two 12V geared motors and comes with a rechargeable battery. It can cover up to 500m2.
You are going to need a 3D printer, rechargeable battery, Arduino and a few other parts to get started.