One of the primary places to look for help is the main Tinkercad website.
Don't be fooled by Tinkercad's simplistic look. You can actually create some sophisticated shapes. The most important thing to remember is that all of 3D design is based on these fundamentals. These are the add and subtracting of basic shapes. You then bend, stretch and manipulate these shapes in hundreds of ways.
Another good tutorial is from All3Dp.com
Tinkercad is a great tool for generating designs from basic shapes. It is not as advanced as other 3D design software, but you can really push it's boundaries with your imagination. The best aspect of Tinkercad is that it is totally browser based. You do not need a high end computer to run this software.
It is hard to create designs with accurate measurements with Tinkercad, but it is excellent for exploring the design process, and creating artistic projects.