Three Ways to Tune

1.  Use a tuner app. A Google search for guitar tuners will turn up an impressive selection of apps for any operating system.  Be sure to choose an app that includes a chromatic mode (that means it can tune to any note). My favorite apps are:
iPhone/iPad     Instuner           (free from the App Store)
                     Windows           PitchPerfect  (free from www.nch.com.au/tuner)
2.  Use an electronic tuner. Clip-on tuners make tuning in a noisy room much easier.  Attached to the headstock they read the instrument's vibration, so they don't get confused by other sounds.  My favorite clip-on these days is the Snark SN5X.  It's bright, has only one mode (chromatic), and works well.  At $10 or so, it's a bargain.

Snark SN5X. 
3.  Tune strings to each other. You can use this method anytime you're playing solo.  Your instrument will sound good, but you may be up or down from the exact design key. 

Follow these steps:
            Fret the top (thickest) string at fret 8 (see fret numbering info); it's before a long space.
           Tune the bottom (thinnest) string to match the fretted top string.
            Fret the middle string at the third fret.
            Tune the fretted middle string to match the bottom (thinnest) string.
Dulcimer Tunings

Dulcimers are typically tuned in a 1-5-1 pattern from low to high.  On a stick dulcimer with no frets pressed the upper (thickest) string sounds the lowest note, the middle string is the fifth note up, and the bottom (thinnest) string is one octave above the thickest string. 

Modern dulcimers are most often tuned D-A-d (thickest-middle-thinnest), though G-d-g, D-A-A, and C-g-c are also used. Our instruments are usually tuned D-A-d. 

( Notation: capital letters denote the third key of that note on a piano keyboard.  Lower-case letters represent the fourth key of that note.  Many sources use numbers to designate the note, so D-A-d would (usually) be D3-A3-D4.  If you're new to this and find it confusing, just email me.)

Traditional mountain dulcimers are strung in the opposite order.  The thickest string is farthest from the player and the thinnest string is nearest.  This arrangement is sensible for an instrument that's played on the lap, because the melody string is the most accessible at the front.