Your best bet is to contact a Key West tackle shop and book a guide through them. Most keep a pool of guides they schedule bookings for and cover a wide-range of speicalites: inshore, nearshore, & offshore, flyfishing, light tackle, ...
The most important thing you can do when booking a guide is to give them an accurate description of your desires and experience level. Some guides are great teachers for novice anglers and some suck at it. Some guides are good with young anglers and women -- some are not. The tackle shop will make their recommendations accordingly. But they can't do it if you don't tell them.
The shop I used when I first started going to the Keys is the Saltwater Angler in Key West.
http://www.saltwaterangler.com I had just gotten into saltwater fly fishing and wanted help with my casting as well as catch some fish. Worked out well...
Another shop to call is Conchy Joe's Tackle in Key West.
http://www.conchyjoeskw.com . Rob is the owner and a good guy.
Tightlines,
PB
Key West in April--Guide Needed!!! 
