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Don't start with a human being on your first try.
There's so much more you need to learn about the tools themselves before you start attempting to project an object inside of your head to a 3D model.
You'll find that 3D art is more science than art, and that the tools you use will play a large part in what you're able to do.
Having said that, if you're unable to do it despite understanding your toolset, then you're just a bad artist.