The official written documentation is at
https://supertuxkart.net/Community on the right side.
Making karts and tracks will require using Blender (a 3D modeling tool) with the SuperTuxKart Blender add-ons.
Here is the install guide. You can ignore the parts about older versions.
Let me know if anything isn't clear: I'm working on improving the documentation so your feedback is wanted! There is also a (work in progress)
video tutorial.
It also requires adding the media repository, which has all the media like textures and sounds that get included in the game. See
this page. Again, if anything is unclear, let me know and I'll guide you through.
The written documentation is currently the best source for learning to make tracks and karts, apart from asking questions here or the chat room. There is an old video guide on YouTube for making a track which may also be useful if you get stuck on a part, although it is in German, and 5 years old (so the parts about installing tools and many of the Blender instructions may be outdated, including important things like textures, but it will show you how to made a valid driveline using the array tool if the written documentation is confusing). "Create Supertuxkart-Track with Blender [Beginner]" by Linux Guides DE.
A useful thing to do may be to download the source of a track that someone has made, open it in Blender, and try exporting it. I can upload one of my basic test tracks, because older track made in Blender 2.79 or below used a different technique for adding materials, so all the materials will be broken.
aka. GumballForAPenny