I learned on DesignSpark Mechanical and if all you ever want to do is make STL files, it's really great and super easy (more so than sketchup). Unfortunately, if you ever want to take the next step and actually manufacture something you will run into trouble since nobody else can read their CAD files. Therefore, I have switched over to AutoDesk's Fusion 360. It's super powerful and Free if you are a student or start up. It's not quite as easy as DesignSpark but it's easy enough and WAY more powerful.