If your data is in Geographic Coordinate System(WGS84 etc..), the units will be in decimal degrees(angular). So I would project the polygon shapefile first to a Coordinate System with Meter as unit(like NAD83 UTM Zones..). Then the buffer distance will be in meters.

With a Quick google Search, find the conversion between Nautical mile and Meter.

12 Nm = 22224 meters

Now Run Vector > Geoprocessing > Buffer give 22224 as buffer distance

An excellent answer is here http://gis.stackexchange.com/a/23863/5850

If your data is in Geographic Coordinate System(WGS84 etc..), the units will be in decimal degrees(angular). So I would project the polygon shapefile first to a Coordinate System with Meter as unit(like NAD83 UTM Zones..). Then the buffer distance will be in meters.

With a Quick google Search, find the conversion between Nautical mile and Meter.

12 Nm = 22224 meters

Now Run Vector > Geoprocessing > Buffer give 22224 as buffer distance

An excellent answer is here https://gis.stackexchange.com/a/23863/5850