It is imperative to know the main string tunings if you want to play a certain song exactly the way it sounds. If your off by just a bit, it will throw off your timing and sense of location on the guitar, and the tabs wont make sense to you. I advise to buy a chromatic tuner, a kind where you can adjust pitch, and can select from notes A, A#, B, C, C#, D, D#, F, F#, E. They run from about 20-30 dollars.

Here are the tunings.

Standard-               E, A, D, G, B, e                          Example- Enter Sandman

Half Step-              Eb, Ab, Db, Gb, Bb, eb               Example-  Fuel

Dropped D-           D, A, D, G, B, e                          Example-  Keep Away

Whole Step-          D, G, C, F, A, d                           Example-  Sad But True
 

Then there are custom tunings that the new metal bands use.

Limp Bizkit (1 1/2 step)-     C#, F#, B, E, G#, c#

Slipknot-                            B, G#, C#, F#, A#, b

It is probably a good idea to have a locking tremolo and locking nut on your guitar to keep the strings tight and ease the tension on them before trying the 2 tunings above. I broke a string from trying to get it to a lower note.