The first step in modelling your idea is to choose a tool. There are many solid modelling applications and all have their pros and cons; we've chosen the ones we like best for 3D printing.
Inventor is a great program for printing engineering prototypes. You can create accurate parts and piece them together in assemblies, perform stress tests, simulate motion, create drawings, renderings, and more. Since Inventor 2015, Freeform modelling provides the ability to model organic, soft shapes, from machine handles to jewellery. The learning curve is steep, but if you're an engineering student you already know how to use it thanks to 1C03. Inventor and most Autodesk products are available free to students through the Autodesk Education Community.
For more information about 3D printing with Inventor, check out our Autodesk Inventor guide.
SketchUp is a great tool if you're just starting out with 3D modelling. It is very easy to learn with the included tutorials and guides, and is very widely used across the web. If you need to know how to do something in particular, there is most likely a YouTube video to help. SketchUp does best with measured parts, buildings, layouts, mostly boxy items or simple revolutions. It is free and can get you making things fast. You can download SketchUp here.
The 123D suite of Autodesk applications contains 8 free programs that focus on creations in 3D space. They are targeted at beginners and make it very easy to create basic geometry with drag and drop simplicity. Check out 123D Design for a straightforward solid modeller based on manipulating primitive shapes, or TinkerCAD for an easy browser based CAD app.
OpenSCAD is "the programmer's solid 3D CAD modeller". Unlike many other applications, it takes your code and translates it into geometry. It makes it easy to create models based on mathematical equations and logic. If you are comfortable with your coding abilities, OpenSCAD is fast and accurate with the ability to create parts ready for 3D printing. OpenSCAD is free and available here.
3DS Max and Maya are applications for modelling, animating, and rendering in industries like film and video game production. They are great for modelling soft, curvy or organic shapes like people and characters. There is a large learning curve because these software packages have a ton of features. Autodesk 3DS Max and Maya are available free to students through the Autodesk Education Community.