Tanya - welcome to our little corner of musical paradise. I'll give you a short answer here, and suggest that you start a new separate discussion in the Beginner Group rather than side tracking Mandy's discussion. Tht way too mor epeople will see your questions and respond to them.
The first thing I suggest is that you read the article I wrote called I Just Got A Dulcimer Now What?? It's an illustrated glossary of dulcimer terminology, plus answers to many beginner questions about the tuning, playing, care and feeding of our favorite instrument.
The article is here: http://mountaindulcimer.ning.com/profiles/blogs/i-just-got-a-dulcimer-now-what
Things to understand... With a given set of strings (diameters) you can only tune the dulcimer up or down a couple of keynotes before things get too tight or too sloppy. Generally speaking, you can only play in one Key at a time. To change keys you must re-tune the dulcimer. Most dulcimers are tuned to the key of D and with the 'normal' string set can tune down to C or maybe B, and up to E and maybe F. Any other keys require different strings. Under most circumstances the Keynote of the dulcimer is defined by the note to which the bass string is tuned. That's the farthest string from you, The middle and melody string(s) are tuned to higher pitches relative to that keynote -- for example DAA, CGG, DAd, CGc. For each keynote the multi-finger chords will be different.