y did i do it?

I was really upset and i was looking for a battery so i could tune my guitar but i saw my dad's pen knife and i took it and cut myself and my best mate now hates me and i fort it wood make everything feel better but it didnt and now im even more upset :(

dont cut youself and tell your friend it was a accident

As Ghostface said.

Problem sorted!

unless you want a load of scars on your body, obsessive thoughts about when, how, and where you'll cut, and all you think about is cutting, and all you fear is being caught(though, in your case i guess that isn't an issue), then don't go down that path. :)

plenty of other options rather than cutting yourself.

Why cutt when you can have fun with your 'wong'

Play your guitar when you're upset, it will calm you down. :)

Play rugby! Take out your emotions in the big hits.

I think it's wrong for your best friend to hate you, I think he/she should've helped you through it.

Say It was an accident, and DONT go it again!

Its dangerous!

I once cut myself when i felt upset once, i thought it made the situation worse, didn't help one bit.
Try doing something else that puts your mind off what ever your sad about, normally if i'm sad or angry i head over to my punch bag. ;P
If your friend hates your because you cut yourself then thats wrong. Make something up like you cut yourself, i said i fell over onto some glass and it cut me when i cut myself.
But like what was said above, don't get into the habit of it.