Start off with Thingiverse or similar. I recommend something like: https://www.thingiverse.com/thing:3553160 There are a lot of models there - those are .STL that you “slice” and send to your printer. There is this: https://www.thingiverse.com/thing:2187167 which is CAD models with the working left in - OpenSCAD in this case. You load it up, generate a .STL and then pass that to the slicer.
I have a large plastic cabinet in my garden for storing a lawn mower etc. The hinges died years ago. I have printed new ones. At least 20kg of plastic waste has been avoided being dumped a lot longer. I am well aware (now) that I should not have bought the bloody thing in the first place!
If you go the OpenSCAD route, you might like this: https://github.com/JustinSDK/dotSCAD - the author sadly passed away recently but his work is legendary in my opinion.
Once you get printing sorted out, then move on to your own stuff. … or not - give it a go! I have a large bag of very strangely shaped PETG experiments that went badly wrong and need recycling.
SO: Next door have got their lights up, why haven’t we? Me: (Enable NodeRed flows for gutter and pergola light strings that switches them on at dusk and off late evening) Right, that’s the missus pacified for a week or so. I should probably get BigTimer to sort that out itself.
Employee: I’ve got all the printers for <customer> monitored using an auto entries card. (Good skills) Me: (Installs an addon that can use VoIP to do text to speech to a phone) We warn off the customer and now they get a phone call from “things” that tells them what consumables to buy and also sends an email.
When I finally get around to sorting out my glasses so I can see what my soldering iron is up to, I’ll get many more gadgets installed. My computer room at work needs a tiny ESP8266 and four 1 wire Dallas SC temperature sensors, a bit of vero board, a resistor, power and probably a buck convertor and a case, which I’ll print.
I adore HA.