First, lets look up on the ways you can tune different bass guitars.

4 String bass: all High to Low or Thin to Thick

Standard: GDAE

Drop D: GDAD

Drop C#: GDAC#

Half Step Down: GbDbAbEb

Full Step Down: FCGD

Drop D/G: GDGD

Raised G: GDAG

Low B: DAEB

Low A#: C#F#D#A#

Low C#: G#C#F#C#

Low C# Dropped To A#: D#A#FA#

5 String Bass:

5 String Standard: GDAEB

Drop B: CGDAE

Drop B/C: GDADC#

Half Step Down: GbDbAbEbBb

Full Step Down: FCGD

6 String Bass:

6 String Standard: BGDAEB

Low B: CGDAEB

Low C: EBGDAE

8, 12, 15, and 18 String bass:

8 String Standard: GgDdAaEe

Full fifths: GdDaAeEb

12 String Standard: GggDddAaaEee

15 String Standard: GggDddAaaEeeBbb

18 String Standard: BbbGggDddAaaEeeBbb

Full Fifths: GdgDadAeaEbe

Full Fifths: GgdDdaAaeEeb

Now lets look at tuning the bass guitar!

Tuning the 4th string:

We are now going to tune the bass to standard pitch. The 4th string is the thickest string, and sounds the lowest. Try to hear if the 4th string on your bass sounds higher or lower than the note you are hearing below. Then try match your bass to the note you are hearing. As you are listening to the 2 notes, try and hear the "crashing" sound that comes from the notes being out of tune. The closer you are to being in tune, the further away the notes will crash. To raise the pitch of a string, turn the tuning pegs that face up counter clockwise. If the tuning pegs face down then turn clockwise. Reverse the direction to lower the pitch. Never turn the tuning pegs unless the note is ringing. Otherwise, you will have no idea how far to turn the tuning peg.

Here is an example to tell if the 4th string is in tune. 4 string bass standard tuning is GDAE. The G string is the highest and the E string is the Lowest.

G|------|
D|------|
A|---0--|
E|--5---|

If those notes are the same, your in tune with the A and E string. Now do the same to the rest of the strings.

G|---------0--|
D|------0-5---|
A|---0-5------|
E|--5---------|

If the 5 and the 0 sound the same, your in tune.

Drop D makes the 4th string deeper. Here is how to get in to drop D:

G|------|
D|------|
A|---0--|
D|--7---|

If they sound the same, your in drop D variations: Drop D down a 1/2 step: From standard Drop D tuning, tune all of the strings down a 1/2 step (like they were 1 fret lower). Drop D down a whole step: From standard Drop D tuning, tune all of the strings down a whole step (like they were 2 frets lower). This is sometimes called Drop C by mistake. Drop C would mean to drop only the 4th string down to a C.

Read more http://www.ultimate-guitar.com/lessons/bass_lessons/bass_guitar_tuning.html