First you need to find a software for your designs. Since you are attending school, you can get free versions of all the programs from Autodesk. I work a lot in Autodesk Inventor. A very handy tool for creating all things engineered. If you think it is too hard to learn, try Google Sketchup. If you download the stl plugin for it, you can make 3d-prints from Sketchup.
Try making cookie cutting shapes to learn the basics. Useful and fun.