There are a lot of ways to find interesting things to print using a 3d printer. Thingiverse, or other stl repositories are abundant with files to download and print on the printer. You can commission an artist to create something for you. Join a Patreon group that creates figurines for your tabletop game. But, sometimes you can’t find what you are looking for online or just want to create your own.
There are many available tools, with more being created all the time. But, there are two that I use consistently.
For functional parts, such as when I create 3d printed widgets to replace broken things around the home or when I create photo stands for my family, I use Fusion 360. It’s subscription based, but it still seems to provide the most tools and provide the ease of use that I’m looking for. In fact, it’s so easy to use that I often have my 10 and 12 year old boys make stuff instead of doing it myself. I look at this as a valuable learning experience for them, especially if they ever end up in some type of engineering role for their careers.
For more artistic parts, I use Blender. Blender has a pretty steep learning curve, but there are tons of online resources available to help with that, including many video tutorials.