I'm not an expert on 3d printing, but I've thought about getting into it too.
From my understanding most printers have their own software (typically proprietary) which takes the 3D model (usually in a generic format like .stl, .x_t, or .step) and uses it to set up the machine parameters and generate the code to run the machine. To make your own 3D models you'll need CAD software and the know how to use it, otherwise you're stuck downloading models or having someone else create them for you. There are some free online CAD systems (like Onshape.com) but I'm not sure if they let you download to .stl or another format for use in the printing software. Professional CAD software is way too expensive for hobbyists.
There's also a lot of considerations in exporting / importing your model (like with the resolution of the solid mesh, stress points in the design) and with its positioning on the print bed (like the grain of the print, where the scaffolding needs to be, etc.) which can get really technical. In a lot of cases you have to tailor the design to be 3D printed (sometimes over / undersizing certain features, no threads, etc.). When you're starting out I don't think that's a major concern though.
As far as the actual physical printer I'd think about the largest part you want to make and get a printer with that size of print area. The larger the area the more it's going to cost.
I'd also look for one with a full frame (U-shaped top bar) rather than the single stick type since there's a big difference in the stability of the printer head, and therefore the quality of the printed part.
The bed type is another consideration which should be chosen to match the material and part size you're printing.
I also would only ever get a printer with some kind of recovery built in for if there's a power interruption. On a larger and more complicated part the print time can be on the order of days, so I wouldn't want to have to scrap a part because of a power blip.
Not an endorsement, but this is the one I'm pretty close to buying: https://www.creality3donline.com/off...ter_p0117.html
Once you're set up you can join an online group where people can send you models and pay you to print them for them, which is pretty sweet. Not sure if it would be profitable for you though!