You need one of the thin CNC type spacer kits - ebay search "cnc singlespeed spacer" :) - will work with any hub then or as above a bag of old cassette spacers. To find your ratio go out on your geared bike on your normal routes and stick to each gear for a loop or a few miles finding what suits you best - replicate that ratio in gear inches on your SS.
You need one of the thin CNC type spacer kits - ebay search "cnc singlespeed spacer" :) - will work with any hub then or as above a bag of old cassette spacers. To find your ratio go out on your geared bike on your normal routes and stick to each gear for a loop or a few miles finding what suits you best - replicate that ratio in gear inches on your SS.