Where do I even start?
3D Modeling Basics Simply Explained
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
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