Can't afford a big-time 3D CAD modeling software program? You're not alone and you're not without solutions. There are free 3D modeling software options, including the very popular but impossible to learn Blender. But Blender just isn't CAD. Each 3D modeling system is best used for certain kinds of designs and if you are designing parts you'll want to check out FreeCAD.
This multi-platform (Windows, OSX and Linux) software system is quite comprehensive, providing parametric modeling, 2D / 3D design and a very extensive system for programmed plugins to do practically anything you'd like to try. You can even embed FreeCAD into your own programs.
FreeCAD is based on several underlying open source software packages, not the least of which is Open CASCADE, which provides the 3D engine for the system. FreeCAD also handles import and export of popular file formats, including STL, most useful for 3D printing.
We suggest you give FreeCAD a try. It's not a Solidworks and it is experimental (as in, expect crashes and miscellaneous misbehavior now and then), but depending on your needs - and budget - it just might be a fit for you.