I got an HO scale Athearn GP-38 from about a year ago?

Model Railroader Layouts and layout building
1

What dcc decoder would fit into it? What is the price? How do you install them?

2

Digitrax # DH123D fits,
here is a website with detailed installation instructions.
http://www.trains.com/Content/Dynamic/Articles/000/000/002/386vdfwa.asp