I would reccomend using only distilled water. You can purchase that almost anywhere. I would also suggest you purchase a hygrometer. Many of the digital ones can be had for under 30 dollars. Wet the sponge and wait for the humidity to rise in your humidor between 65 and 70%. Once that level has been acheived consistantly place your cigars in the humidor. Open at least once a week to check the humidity level. When you notice it dropping below 65% remoisten the sponge. Only moisten it. It isn't necessary to completely saturate it.

Hope this helps.