Search to free 3d cad, Try as many as you can and watch youtube clips to learn how to use it. You will quickly find which one is easiest for you to use.
Just a warning that sketchup isn't the best for 3d printing.
As gpb01 mentions Tinkercad is simple for starters.
once you get started then read THIS guide.
Autodesk's 123D design is another freebee which works well and easy to learn - well a lot easier than Solidworks 
. I agree with @Labern, Sketchup has its moments but seems focussed on introducing mesh errors which stops the model being printed
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gpb01 23
Quick answer ... with a 3D CAD
Going back to being serious, what is your "experience" with 3D design? If you are a "beginner", it is probably easiest to start with something simple ... e.g. you can try Tinkercad ... is on-line (you do not have to install any software), is quite easy and is "free"
Guglielmo
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