First you have to have 4 fleet modules ($20) and 20,000 fleet credits. Then you have to be in a fleet who's shipyard is of the appropriate tier, has ship provisions available, and are of a fleet rank that has permission to use those provisions. If you have all of those, then you can go to a shipyard on the starbase or ESD and acquire a fleet ship. Note, this only unlocks it for that character, not account-wide like c-store purchases do.