I use FreeCad. Tinkercad as mentioned above is much easier, but the generated STL files have a small resolution and the printer cannot really show what it can do. If you don't want to do CAD for the time being, you can find quite a few parts on thingyverse.com that you can simply print out. The printer software Flashprint also offers a few simple ways to combine, cut and scale parts.

P.S. Fusion 360 is very good software, but in my opinion it is not free, you have to buy it.

To get started, it is advisable to look at tinkercad.com. There are also good tutorials here that make it easier to get started. There is also an extensive library where you can access ready-made constructions. -> Just use the search function of tinkercad.

As Voodoo83 wrote, there is Autodesk Fusion360 (for personal and Hobby use) from the same company. This software needs a certain introduction. But here, too, there are tutorials that make it easier to get started.

In general, there are many 3D plans that can be used and even modified for free. The search service Yeggi.com is very helpful here.

Be brave!

I use Fusion 360, there are many tutorials on YT. It's relatively easy to use and it's free. You can also create very complex shapes.

I do everything in Sketchup. Export .stl and then read it into the slicer. Works great!
I've already tried other programmes...all far too complicated compared to Sketchup.