In order to gain acces to the Onshape, please complete the Onboarding found in the link below. There you will be walked through how to set up your account and be able to request access to our Enterprise.
To get you familiar with the controls of Onshape, you will be guided through how to design this storage cube. This will have you creating sketches, extruding them into 3D space. You will also learn to remove material, and get familiar with lesser known tools like the mirror and pattern tool.
If you finished the design walkthrough above, I would encourage you to try some of these challenges. They range in difficulty, and will have you using more tools. It is encouraged to try a few of these in your own time.