Depending on your yeast but usually for a California Ale Yeast like WLP001 or Wyeast 1056 or Safale S-05 it would be 67-68 degrees F. As the fermentation starts up, the fermenting wort heats up itself so try to keep it under 72. But shoot for keeping it in the high 60's and you will be fine. Fermentation temperature control along with pitching the right amount of an activated yeast are two keys to successful fermentation. Good luck, happy brewing and catch the bug!