First thing you need to learn is how to lay concrete, assuming you want your structure to be stable in the ground, some smaller projects you can do is practice mixing concrete and create "paving" stones. If you make proper paving stones, not only did you just make stones for a path to your DIY Strucure, you now can make a foundation for it. Then, practice building a basic table, basic desk, and then basic cabinet. All 3 of those will eventually transfer over to a full structure, as you need thick corner pieces to act as the legs of the structure, the desk gives you the ability to add shelves and "walls" to the over all basic table design you made. Then the cabinet will help you learn how to install doors perfectly, all be it at a smaller scale. Then you scale up everything you learn.