Getting into 3d design is much like getting into any other type of design work. You download some software and start playing around with it until you figure out how it works. That may seem like a simple explanation, but that's really the best way to do it. Often times, what you need to do is start out with some google searches. Browse around and figure out what it is you want to build and see if anybody else is doing something similar. Then, figure out how they did it and maybe emulate a little bit.
Here, at Angled, we recommend, to people who are working with 3d printing for the first time, a software called Tinkercad. Tinkercad is a very simple online software, made by Autodesk, that allows you to drag and drop 3d shapes and dimension them so that you can make correctly sized parts. It is a very user-friendly software that is easy to learn how to use, so there is no digging through menus trying to find the tool that you need. What you see is what you get. This allows you to learn the basics of manipulating 3d models, without being overwhelmed by a complex software. Then, if you happen to have a 3d printer, you can actually print your designs, see what comes out and learn that process a little more, as well.
As you become a little bit more advanced, you can move into true CAD packages. CAD stands for computer-aided-design, which is 3d modeling inside of a computer. For those types of things, you're probably going to want to go with either Autodesk, generally Autodesk 360 or Autodesk Inventor, or Solidworks. However, all of these software's can be reasonably expensive. Autodesk 360 is typically around 100 to 300 dollars to purchase a license, but you can occasionally find promotions to bring that price down a bit. Solidworks sometimes has similar promotions, as well. Although these software's are not cheap, they are the industry standard software's that you want to work with if you want to do true high quality 3d modeling.
For more information, check out our Youtube video down below that covers this same topic. Happy designing!