We use Computer Aided Design (usually known as "CAD") to design our robot's systems before we start making parts for the robot. CAD is a useful tool during the design process because it allows you more easily modify parts if you realize the design needs to change. If you made a part out of metal and then realized that the hole should have been shifted a 1/4 inch, it often requires remaking the part from scratch. In CAD, this same operation of moving a hole can be done in seconds.
The trade-off to using CAD is that building the robot virtually requires an investment of time and a different skill set than doing physical fabrication. The tutorials listed below will help you build up that skill set.
Ask Ryan Cahoon how to get the Solidworks installation and license key
Intro videos from FRC1538: https://www.youtube.com/playlist?list=PLMqx2oBm1a-Da_KlAbm5LjNYQ92GDnpIb
Simbot Solidworks Series from FRC1114: https://www.youtube.com/playlist?list=PLG_KOHBuXHNfpa1N23ZJfAHsCVbe3fpXG
Creating box tubing (we use this as a basic building block in many of our mechanisms): https://www.youtube.com/watch?v=tVnP6xuqd8w
Importing and assembling parts: https://www.youtube.com/watch?v=qEk-iQN47vY
Solidworks Tutorial Videos (walkthrough of creating an entire robot for the 2010 FRC season): https://www.youtube.com/watch?v=ogKYHcprYlU&list=PLiKqXuECiKNJaUpNv3ZW-gEKDxfWgShwG
Videos from FRC Team 973 (West Coast Drive, Elevator, and some more general tutorials on CAD skills): http://www.greybots.com/videos.html
Designing the T-Shirt cannon superstructure: https://www.youtube.com/watch?v=wI2vit7VdKc
These are professionally-produced training series that are designed to teach every aspect of Solidworks. Watching them in entirety is not necessary to get started with Solidworks, but they may be useful if you're interested in learning advanced features or learning how to do things "the right way."
SolidWize: https://solidwize.com/training/ Username: ryan.m.cahoon@gmail.com Password: team766bears
SolidProfessor: https://app.solidprofessor.com/lmsapp/#/library/list/1 Username: ryan.m.cahoon@gmail.com Password: Des1gn
We store our CAD and CAM design files on a Subversion server. Ask Ryan Cahoon for access details
The team's computers use the TortoiseSVN client https://tortoisesvn.net which makes it easier to access the repository by integrating with the Windows file browser. Here's a tutorial on how to use TortoiseSVN: https://www.developer.com/open/beginners-guide-to-tortoisesvn-the-windows-subversion-client.html