If you have a mathematical mind then CAD is the way to go and Fusion 360 (hobby or paid) is a very good way to go.

If you are more artistic minded then art focused 3d software is the way to go. Blender 3d is a free open source excellent option though a high learning curve. Zbrush ($$$) is the industry standard for digital sculpting.

Got my 3d printer a year ago and also a big kid with it.

https://www.youtube.com/watch?v=1egND_XZPjg