It really depends on what kind of camera you have, and what it is capable of.
The lower the ISO, the Longer the Exposure = Best night picture.
Best thing would be to go to flickr.com and look up "Night photography", find pictures you like, then look at the properties of the pictures for their iso/exposure/f-stop to get a feel.
for night ISO should always be turned as far down as possible to reduce or remove noise.