When I was thinking about buying a singlespeed bike I just left my Trek in one gear.

Use a gear ratio calculator: http://home.earthlink.net/~mike.sherman/shift.html

Count your teeth, probably on your big sprocket, then enter your numbers----chainring and cog teeth. Pick a gear that's somewhere between 60-70 inches and leave it there.

A few days riding in that gear will give you a good indication of how you will like life with a singlespeed.