123D Design, by Autodesk, is an entry-level CAD program that allows you to create your own 3D printable designs. The application is designed for users who don't have CAD experience. You begin with basic shapes (e.g. blocks, spheres, wedges, etc.) and can then add more complicated, pre-built items like engine parts, gears, lighting fixtures, and more.
You can use all of these shapes and items to design and build your own 3D creations. After you're done, 123D Design allows you to Save a Copy, Order a 3D Print, Send to 123D Make and more.
The 123D Design application is available in 3 different forms: online, an app for your iPad, or you can download the software for your PC or Mac. The apps are very similar with the exception that the download for PC or Mac offers a few more features including the Print Utility. The 3D Print Utility optimizes a model for 3D printing saving you time and print material.
You will need to create an Autodesk username and password regardless of which 123D Design app you choose. With your username and password, you'll be able to save your designs and send them to other Autodesk programs. You can save your project as private or public. Saving your project as public shares your design with other Autodesk users. You can also download their designs to modify and 3D print.
If you'd like to 3D print a 123D Design project at home, you'll need to save your project and then download your project as an STL file. You can use the STL file to print with your home 3D printer.
Autodesk offers some video tutorials and a user forum to help you get started with 123D Design. Once familiar with 123D Design, you can use the software to create basic 3D models. This is a good program to begin with if you are new to 3D designs and printing.