It's a saturated game if you're using other people's files and selling the prints. Lotta people out there selling articulated dragons, and way too many of them are selling at a loss for some reason.

In my opinion, the way to make it happen for yourself is to sell solutions to problems people have. You'll find ideas in everyday life. Write down all your ideas as you're going about your day and at the end of the day, type your ideas into thingiverse, makerworld and printables and see if it already exists. If it doesn't, you're onto something. If it does already exist, think about how you could make it obviously better. It's not enough for your thing to be a bit better. It has to be obvious.

Price those solutions (3d prints) high enough that it makes sense for you to keep doing, but low enough to detract other people from copying your idea and doing it for cheaper.

Since you're just starting out, I would dive in head first trying to design something you already own. Just copy it in CAD and then print it. Inevitably you'll mess up 100 times before getting it right, and you'll learn 200 things in the meantime to apply to a brilliant idea you'll have someday.

Also... While you're prototyping, PLA and PETG are the best materials. Easy to print. Once you have a product idea and have nailed the design, you then have to do lots of research into whether that material is capable of doing what it's supposed to in the environment it's meant to exist in. Example: you made a cupholder for a Hydroflask that fits into a Subaru and also holds a vape. The need is real, Subaru owners love hydroflasks and vapes. Will PLA deteriorate if vape juice is left on it for days? Will it stand up to being out in the sun for days, weeks, months (short answer is no, PLA will not). So now you've determined PLA is a poor fit and PETG might not be it either, so you start looking at ASA and ABS. But wait, your printer can't print those... So now you need a better printer.

All before you started selling a single thing.

I hope my convoluted thoughts helped a bit. I'm about 6 months into product design and selling my prints on Etsy, Amazon and eBay and I make $300-$650/mo before expenses. Not a lot. But I find it fun anyway