If your dulcimer is a “traditional” one….
…with no extra frets, then you’ll need to use and play in different tunings in order to play the songs you want to play. The typical major key songs will require the 1-5-5 (often D-A-A) tuning for which the Ionian scale starts at fret 3. Typical minor key songs will require the 1-5-b7 (often D-A-C) tuning for which the Aeolian scale starts at fret 1. Mountain minor songs will require the 1-5-4 (often D-A-G) tuning for which the Dorian scale starts at fret 4. Mixolydian tunes like Old Joe Clark will require the 1-5-8 (often D-A-d) tuning for which the Mixolyidan scale starts at fret 0.
If your dulcimer has a 6-1/2 fret…
…you have the option of getting two different modal possibilities from each tuning. For some people this is a big benefit because it means less retuning, but then remembering when to use or avoid the 6 or 6-1/2 fret.