A cloud-based 3d CAD/CAM package. It's free for students, hobbyists, and startups making less than $100k/year. Extremely powerful parametric design software package. Highly recommend. You can find tutorials on YouTube as well as online courses on websites like Udemy or just wing it and learn by doing.
Slicing software. I can say much about it because I don't have much experience with it. I encourage everyone to try slic3r and Cura so they can decide which one meets their needs the best.
This is the slicing software that I use. It's less complicated than Slic3r but still does everything I need it to do. This really comes down to personal preference, though. Also, some models are better sliced with Cura and some are better sliced with slic3r.
A browser-based 3D design and modeling tool that's easy to learn. Not as powerful as Fusion360 but the learning curve is much less steep.
"Swiss Army Knife" for 3D meshes. Use it to reduce the number of triangles in a .stl file before you import it into Fusion360 for editing.
This software is a little different than most cad packages. It's code-based so if you're a programmer, it might be worth checking out.