To tune to your piano, the bottom string (the skinny one) is the same as the E above your middle C.
The strings are (skinny to fat) e, B G, D, A, E
They span 2 octaves from e to E, so start with that E above middle C, and walk your way down. OR, once you have the bottom string in tune, you can tune it to itself. this is hard to explain with out a picture. You could do a search on Google, or something.
But, yes, and electronic tuner is a good thing to have.