We just moved to Guam a little over a month ago. My husband is Air Force We live about 15 minutes from Andersen. There are a few stores on the island to purchase furniture. Most furniture stores are kind of expensive though. If you will have Exchange privileges, then you can get just about any furniture through the exchange either on Andersen or Big Navy on the other side of the island. There is also a Kmart close to Andersen that has some furniture and appliances.
Depending on the size house you are looking for, there are a few really nice neighborhoods. Most 3 bedrooms run between $1800-2400, depending on location. Check out
Martin W. Howard this website for a better idea of house on the island. You'll want to look in the villages of Yigo, Dededo, Mangilao, Barrigada, Tumon, and Tamuning to stay somewhat close to Andersen. Yigo is the closest village to Andersen, then would come Dededo. There are a few nice new neighborhoods in Dededo, like Perez Acres, and Paradise Estates, definetly check those out. We worked with a company called GuamPCS (.com) when we moved here. They are part of The Real Estate Professional (an agency on the island) they were a wonderful help in finding a home!
I don't know much about mountain biking here. There are a few mountains (what I would call big hills), one pretty big one on base that the troops run every so often.
Anything else you wanna know feel free to ask! We couldn't find much information when we were moving here, and it was such a pain!