In order to be able to 3D print, what you print determines where you got your file from. 3D printers use a file format called .stl. stl files are exported out of 3D CAD programs. So most likely you will have to learn a 3D CAD program.
Thingiverse has lots of pre-made .stl files that you can download (most are free) and send to your 3D printer and print right away. But I think for most of the parts or objects you will want to print, you'll have to create them yourself using some kind of 3D CAD program.
I use Sketchup online which is free. Recently I've been learning openscad (open s cad) which is free and to use it, you write script-like statements to define your 3D parts or objects. Being a programmer I was comfortable with that way of working.
FreeCad is a great free 3D program and I spent some time looking into it recently. It works more like traditional CAD programs but it is buggy. Everyone admits to that. You'll want to try both and see which one you like working with. If you use FreeCAd, save your work often!
Fusion 360 does have a free version. Be aware that while it is a capable program, it is a massive program and over kill for most 3D printer hobby projects. It will work and do the job but it's like someone learning to fly using a Boeing 747 instead of a traditional two seat propeller airplane.
Your 3D printer will come with a Slicer software program which is what you use to load in the .stl file. You'll do a 'slice' operation which will produce a .gcode file. You run this .gcode file on your 3D printer to accomplish the actual 3D printing. You can connect a USB cable to your 3D printer and run your slicer program on your PC and send the .gcode file to your 3D printer. Or you can load the file on an SD card and insert the SD card into a slot on your 3D printer and interact with it using a LCD screen to get to print.
I recently bought my second 3D printer, an Ender 3 Pro from Creality:
https://www.creality3d.shop/collections/ender-series-3d-printer/products/creality3d-ender-3-pro-high-precision-3d-printer It's a very popular 3D printer with lots of support online and Youtube, etc. I find the quality of its output to be extremely high. Much better than my first 3D printer which was a Prusa type printer.