3D models for gears can be created different ways - each way has pros and cons

Calculate parameters for gears manually - diameters of gear wheels, angles of teeth, etc. Create a 3D model in CAD software. Print models, test, correct, repeat...
Measure existing gears and use these parameters to create 3D model in CAD in the way as above. It's not easy to find gears similar to needed (even when they can be scaled)
Take the photo of existing gears (e.g from a toy car), print it scaled, put on the display and draw the 2D geometry in CAD - to reproduce gear's shape. This is more easy than in previous steps, but also required to find similar gears (or at least their picture)
Use graphics software with build in gear-generation features. E.g Inkscape (graphics editor with extension which renders a gears), FreeCAD (free CAD software), etc.
Use a program which generates geometry of gears by specified parameters - this way is describes in this video instruction.

Software used in this instruction:

Gear Generator - this is not free software. It allows to generate 2D geometry of gears and export it to dxf-file
PTC Creo Elements/Direct Modeling Express 6 - this is free software which allows to create quick and easy mechanical models for prototypes. This product allows to create models the way similar to solid modeling - but more simple. Version 6 has a great new feature - export 2D geometry on a workplane, this feature is used in the instruction.
Netfabb Basic - this is free software which allows to fix STL-geometry or rotate parts.
KISSliser - this is free software (with additional features available in purchased version). Prepare STL-geometry for 3D printing - slices and translates to G-codes.