This is an easy to use, browser based, 3D design and modeling tool, that is free. There is a limit to the size of the file that can be uploaded, which is 25 MB, but anything under that size can be altered using this website. The main purpose for this in the imaging lab is to hollow out models in order to save on the amount of material the 3D printer will use to create the model.
Help with Tinkercad: Tinkercad Lessons
If you are having problems understanding how to use Tinkercad, on their website they have posted simple lessons that will walk you through the basics of using it.