Some songs for bass require that the guitar be tuned to a different set of pitches that that of standard tuning.  Some ways for a four string bass guitar to be tuned include (from low to high):

Standard tuning: EADG
Drop D: DADG
Drop C#: C#ADG
Drop one half step: EbAbDbGb (b= flat*) or: D#G#C#F# (#= sharp**)
*A flat note is one note lower than the standard note of which it is.
**A sharp note is one note higher than that of a standard note of which it is.
Drop Full step: DGCF

For five string bass guitar, standard tuning is as follows: BEADG
As for six string bass guitars, standard tuning is: BEADGC

To tune your bass, you may use a chromatic guitar tuner, online tuner, or tune it by ear.  If you are looking to tune by ear, the following should help:

G|---------0--|
D|------0-5---|                “0”= open string
A|---0-5------|                “5”= fifth fret
E|--5---------|

The fifth fret of the “E” string is the same note as an open “A” string, and the same trend continues for each other string. 

To tune for drop “D”, the seventh fret of the lowest string should be equal in pitch to the open “A” string.  This is because the low “D” note is two notes lower than “E”, so therefore it should be equal to two frets higher than what “A” is on the “E” string in standard tuning.

This link should help tune your bass: http://www.tunemybass.com/

Written by Nick T.