Here's one of the best tutorials that I've found; it's set up in Pro/E Wildfire 2, but as long as you can get Inventor to solve the equations, I think that it could work. It's heavy on math, but you can scroll down to the solved equations, and implement those without necessarily working through the theory.
This will get you proper gear geometry, suitable for scaling to any size, and for cutting on a machine (such as an EDM, or a maybe even a waterjet or laser). Depending on your computer, and Inventor's capabilities, it may also take a long time to regenerate that sketch, every time you load the model.
In practice, you don't necessarily need that sort of precision for many jobs—it's really up to you, depending on what you need it for, and what tolerances your equipment can hold. In the event that you can deal with slightly imperfect geometry, try
GearGen (
or here), which will output a gear with small arc segments, rather than a continuous curve, in .dxf format. This can be imported into Inventor, and extruded.
In any event, for simply
drawing the gear, you can just pattern an approximately correct tooth (using an extruded cut) around the circumference. It's also common to just model the gear as a cylinder with the pitch diameter.