Tinkercad is a good way to get going quickly and if you haven’t done any 3D modelling previously I’d strongly recommend starting there. It’s extremely easy and intuitive and you’ll be making something within the hour. Whatever you make can be exported as a printable .STL, along with other formats. You can do quite a lot with it but will eventually run up against its limitations.
The next step on is a key decision point, because whichever app you choose, you’ll end up investing a lot of time and effort on the learning curve. There is a junction of sorts here: the softwares are broadly either CAD, aimed at ‘engineered products’, or ‘artistic’ apps like Blender. The former are (in my view) better for making precisely dimensioned model parts, the latter for sculpted type objects. (Though sooner than you think you may need both).
You’ll also want to decide whether you’re willing, down the line, to pay to use the software. CAD can be very expensive. If like most of us you’re therefore only interested in apps that are free, there are quite a few out there but I think the main free ones are Fusion 360, Solid Edge, FreeCad, and Sketchup. I can’t really talk to the latter three; when I was in your position a couple of years back, I gave all of them a try and preferred Fusion 360. Contrary to what some folks think, it’s still free for hobbyists, the only real limitation is a slightly irritating limit on the number of files you can work on simultaneously. But I think all of them are very powerful tools that will far exceed your needs.