Lowe's, Home Depot and DIY Network all have information about installing a gas water heater for homeowners. This is a fairly basic plumbing task that can be done by many people with the proper instruction.
Lowe's offers a step-by-step list of instructions for those seeking out information on installing a gas water heater. There is also a video available.
Home Depot also has a step-by-step instructional page for installing a water heater. In this case, it shows pictures for each step to help installers along. DIY Network also offers images to help with the installation of a water heater. For some steps, there are several photos to make things clearer.