Buying a camera won't make you a photographer. Owning a guitar won't make you a musician. Having a copy of Metasequoia won't make you a card designer.
All these things are tools that help you do a job. While they are powerful tools indeed, they are worthless unless you first know some basics about photography, music, or card design.
In my opinion, the best way to learn to design is to start without such powerful tools. Trying to use a program like Metasequoia to design a WWII fighter (or whatever) as your very first model would be like trying to learn to drive by slipping behind the wheel of 747, intending to fly solo to Paris on your first try -- the basics that you need to learn first will be completely overshadowed by the sheer complexity and capability.
My suggestion is to stat with something very basic, and design it with a ruler and pencil. You need to get your mind around breaking 3D objects into sub-assemblies that can be produced with a nearly 2D material that can be printed on one side. First, get *that* down.
Once you begin to feel comfortable designing simple things, you can start exploring how to do that more easily using an application. Then you can begin to use the application's capabilities to enhance your growing skills and ideas.
Like any skill worth learning, it won't happen overnight. It takes concentration and practice. Study your results to learn from your mistakes and successes. Soon you will find yourself designing increasingly complex objects as you build your experience and knowledge.
Good luck ... and have fun!
