There is a lot of information you are leaving out. Do you know the maker of your dulcimer? If not, there should be a label visible inside one of the sound holes. If there is a label, there should also be a serial number and/or a model number. Then, armed with that information, you can contact the dulcimer maker and request a “string chart”, which tells you the tuning and string gauges. If there is no label, or your dulcimer’s maker is no longer active, then that is a problem. Did you buy it used? Would the person you bought it from be able to help you? It’s easy to measure the string gauges with a digital caliper. Then, given the length of the string, and figuring in normal tension, it can be determined what the proper tuning for each string should be. But you probably need some more experienced help for that.
Perhaps you already know your tuning. Then, is the problem you are having something to do with available instructional materials? Most dulcimer books are written with either 12/11 or 15/14 configurations in mind. The biggest difference, aside from the obvious, is that the 12/11 generally starts with a G as it’s lowest string (unless it’s Michigan tuning, then the lowest note is a D), while the 15/14 starts on D. Obviously, this is critical information to know! You can’t play it if you can’t tune it properly! I am just giving this an educated guess, but It is very likely that your dulcimer is either configured like a 12/11 with additional notes, or like a 15/14 with notes missing. One of my dulcimers, for example, is a 14/13. It is configured like a 15/14 with one missing course on each side, resulting in one missing note on the base side and two missing on the treble side. The dulcimer designer chose “missing” notes that are seldom used anyway, so 99 percent of the time I can use 15/14 music. If I run into a note that I don’t have, I just have to adjust. This is not an unusual situation by any means. There are a lot of different configuations out there. 16/15 is common, and upper line dulcimers are often bigger, and sometimes extra bridges are added for chromatic and extra bass notes. Again, the people who write the books stick to the most common configurations. You just have to learn to adapt and substitute if you have a layout that is out of the norm. It’s an opportunity for creativity! I hope all of this helps. Please keep us updated and let us know what you found out about the maker of this instrument and it’s tuning.