Supposing that your computer is on a different LAN (private network) than the one on which that person you want to share with is, you will have to set Port Forwarding on your router. You will also have to set up a password for the share so that only that/those person/persons you want to share with can access the file(s) /folder(s). Of course the best solution (most secure but not the cheapest one) would be to have a file server/NAS running over SSH protocol.