It is quite simple to teach your dog to touch a bell with his nose. You can then hang the bell from a doorknob to let him ring when he needs to be let outside.
Begin by holding a bell on a rope at your dog’s nose level. It’s best to hold it within just a few inches of the side of his face. Most likely your dog will either turn to look at the bell or investigate it with his nose. As soon as he does either, Click and Treat! If your dog has not actually touched the bell with his nose that is ok, just keep clicking for any look towards the bell, this will teach your dog that the bell must mean something! After several of these looks require him to actually touch it before clicking.
After your dog is touching the bell reliably you can begin to add the cue word “Touch” or “Ring It”, etc. You can also begin to move the bell further away from his face, requiring him to take several steps to the bell in order to ring it. When your dog knows the cue word you can then hang the bell from a doorknob, at his nose level, and ask him to touch it the first few times for a treat and then the reward can just be the door opening up for a trip outside! (Note: An intermediate step that you can try is to throw a treat outside and re-close the door. Then ask your dog to touch the bell and when he does make a big deal out of it as you release him out the door to get the treat.)
After the above training steps, any time you think your dog needs outside, ask him to ring the bell before you open the door for him. Make a big fuss with a lot of praise when he does this while at the same time opening the door. Try to do this every time you put him out.
In several days to a few weeks your dog will probably try going to the door himself and ring the bell when he needs out. The first time he does so and several times afterwards, make a big deal about it! Run to the door praising him for the bell ringing and let him out! Once your dog “knows it” you don’t have to make such a big deal about it but still let him out quickly as long as you believe he really needs out to go potty.