First of all you can do this.
Go to
http://www.solidworks.com/sw/educati...1_ENU_HTML.htm
There are tutorials built around a FRC robot. Most important-- make a good frame. This is the foundation for your robot and will give you the steps to learn good assembly technique. It's not just about the model, it is about skill too.
The tutorials in SolidWorks will also help you. If you decide you want to use a custom tubular frame, go through the tutorial on weldments. This uses a 3D sketch and the cross section of your tube - very nice technique and will lead to your cut list, no scrap.
Learn how to add materials. Material will dictate mass and in the assembly you can sort by the components that weigh the most.
Also, there might not be sustainable design in your robot, but SolidWorks Sustainability can really help you play with the material properities of different components for the "what if" scenerio.
When you first learn SolidWorks, go through the tutorials - step by step. Then go back and made the parts again without looking at the directions. Try to make models on your own.
Also, check out my blog at
http://blogs.solidworks.com/teacher. Engineering students post models of what they have created - just for fun. You have got to have fun. Marie