This guide will introduce you to some of the basics of 3D design and working in 3D space. It will then walk you through a tutorial on designing simple objects in Tinkercad, a browser based 3D modeling program.
Remember when designing that you can make anything on a digital canvas, but not every design can be translated to a 3D printable object.
A few rules of thumb to keep in mind when designing are:
All 3D models will have three measurements to them: height, width, and depth. These correspond to the Z, X, and Y axes.
These axes indicate both the measurement of the model and the direction in which it may move in 3D space. For example a model can be 15mm wide, and also move 15mm along the X axis.
You will need to keep all of these dimensions in mind when editing, especially if you want your object to remain proportional. Residing one dimension without resizing the others, will result in a model that is skewed and deformed.