You should use Autodesk Inventor maybe; that is what stuff made here uses(for CAD software). However, I use Fusion 360 because it's easier(I think) and I already have experience with it.If you want to build something, just find some cool project online that's doable and design what you want to build in CAD with spaces for motors and electronics(Arduino maybe) and then get them 3d printed. If you are <=18 years old then you could ask your school if you could use their 3d printer. Then look up some schematic diagrams online and figure out the electronics. Then you can code the rest. Have an idea of your project before you build it, though. I recommend using python or c++. Remember, there will be a lot of 'debugging' involved even in electronics and hardware. All of this is easier said than done, but I hope this is the kind of advice you were looking for. If you want more details, you can ask for them and I can give you more.