If you are a student you can download Autodesk Inventor Professional for free. Inventor creates gears for you simply by filling in parameters in a dialog box (what are the parameters for your gear).
Do you know how to create a spur gear in AutoCAD? (it is usual to start with a spur gear before moving on to more complex gears)
If you must do in AutoCAD rather than Inventor it would be better with AutoCAD 2010 which allows parametric dimensions and constraints.