I'm a fan of the "Managing the Waterway" cruising guides. They have a Florida Keys guide that we used to find good anchorages and services on shore.
If you want to take a view of the "Cruising Adventures of Wild Blue and Crew" in the Grand Adventures forum, the first page there starts with some of our time in the Keys. We enjoyed the area a bunch; have been there several times.
January can be cold... or it can be lovely. More likely, it will be some of both. When a norther blows in, you want to be in a protected anchorage or at a dock. Fortunately, there are plenty of decent anchorages... you just watch the weather to see which way the wind will be blowing and then decide which side of the Keys will provide the best protection. While I haven't started there, I think Marathon is a good starting point, especially if you just have a week. You can spend a little time there and learn a lot about the cruising lifestyle... plenty of live-aboard folks there, especially in the winter.
Stay flexible, go do some of the touristy stuff, and mostly have fun. Oh, and keep us posted.
Best wishes,
Jim B.