Hang the Christmas "jingle" bell or two on a ribbon on the door knob. Make sure its low enough that your dog can reach it. (you can pick them up at a craft store).
Every time you take your dog out the door - take his paw and ring the bell with it. Then immediately take him out. Eventually he makes the connection. In the future, any time you hear him ring the bell - go straight to the door and open it - even if he does not go out. Reinforce the lesson that ringing the bell gets you to open the door.
You can also purchase a doggie door bell that sits on the floor and they push it with their paws.
I taught my dogs to sit at the door to let me know they want to go out. To do that - teach your dog "sit". Then every time you take them through the door - have them sit. Immediately open the door. They learn that going to the door is the sign. Then, any time you see your dog at the door - get straight up and open it. One of my dogs like to go to the door so I'd open it....just so she could look outside. I'd open the door and she'd stand right there and look. 5 min later - shes at the door wanting me to open it again. As frustrating as it is, open the door - you want to reinforce the sign - especially in the early states.
Good Luck.