Many dulcimer players learn songs by ear and others learn with Tab. Tab is a form of musical notation.  Most dulcimer players learn songs by a combination of learning the tune by ear and by referring to the Tab ...

Tab is available from a number of sources, some free some you need to pay for.  Please note that, in general, the tab you pay for is far better, both in quality and accuracy,  than any of the the free tab you can find. 

To play by numbers, so to speak, using Tab, the strings of your dulcimer are represented by the three lines below the musical staff and notes.  The top line will be the frets of your bass string (farthest from you), the middle line are the frets to play on your middle string, and the bottom line represents your melody string (closest to you).

When learning a new song most people focus on playing the frets on the bottom, melody line.  

Look at your fret board.  It has a series of large spaces and small spaces.  Now match that to one of these typical turnings shown below for your dulcimer ... 

Because most dulcimers are diatonic advanced players often re-tune their dulcimers to match different keys.  You can for example, easily re-tune your DAD dulcimer down a full step to "CGC".  This allows you to play tunes that are Tabbed for "DAD" in the key of C instead of the Key of D.  

Note that when re-tuning, all the notes on the fret board change!  Because dulcimers are frequently re-tuned, most dulcimer players play by the fret numbers that are shown in Tab and they don't bother memorizing the note positions on the fret board.  Yes, eventually, advanced players figure out the notes in the tunings they frequently use but it can be a distraction for beginning players to worry about the notes at each fret position until they settle on the specific tunings they will be using. Therefore - beginning players focus on playing by the numbers that are in the Tab below the musical notation ....

Tab Sources:

Marlboro Dulcimer Gathering Songs

Other Free Sources:

Not so free ... but absolutely fantastic :

(If I am missing someone's awesome books  ... please contact me!)