In the end, you can tune it whatever you want (depending on the strings you use), but the standard 5-string consists of a regular 4-string tuning plus a low B (a perfect fourth below the low E string).
The fifth slot in a 5 string bass can be used for a low B string, or a high C string. Most people use a low B. For me, I find it's easier when playing in E or Eb.
