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It is a mandolin banjo and is tuned like a mandolin or violin, pairs of strings
in unison G D A E from biggest to smallest, 7 frets apart.

Mandolin strings will work if they are long enough. Bridge position may be
off (usually is). It can be adjusted by comparing the chimed and fretted
notes at the 12th fret. Should be exactly the same. If not, move the bridge
closer or further and get them to coincide. When they are the same then
you can go ahead and tune it up and probably should then check again as there
may be some flex or give in the instrument and placement is fairly critical
to good intonation.

There is a good variety of electronic tuners available in a number of different
brands. I prefer chromatic ones because then you can tune anything with them.
For bass and autoharp I use a clip-on pickup with my tuner. They can be gotten
from places that sell hammer dulcimers. Tuners continue to get cheaper and
cheaper, like some other electronic equipment.
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George Downs, Bartlesville, Oklahoma, Central US