To teach a dog to bark, first you need to teach him to "speak". Then each time you go to let him out, you have him "speak". Eventually he will learn to "speak" by the door to go out.
to teach him to speak you need to find something that will make him bark, some sort of trigger..a toy, a noise, ringing the doorbell, knocking on the floor, anything that will make him bark. Then tell him to "speak' and show him/do the trigger. When he barks, tell him ...goood speak...goood speak and give him a treat. Do this over an over until he speaks on command. Then start having him speak at the door, and when he speaks , open it immediately.
Many people teach their dogs to ring a bell at the door, that may be easier. Hang a bell on a rope, or buy one of those bells like on a counter at a store, and each time you open the door ring the bell, then show your dog how to ring the bell by "touching" it with his foot.