Oh goodness

You should tell your best friend and you should tell an adult and get professional help. There could be a number of things that is going on and it is best to get help now before it gets too serious.

id say use distractions, go on a hike or a run, play music, write, take a bath, light a candle, read a book, smoke some pot (it can be very opening and awakening), do art, read, meditate, listen to music, volunteer somewhere, talk to a friend, spend time with a fun sibling, take a shower, etc! there are many ways to change your thought process. just be committed to happiness and it will come to you!

you should probably tell a professional. My sister used to cut herself and in the end it really wont make you feel better. The only way to feel better is to talk your feelings out through therapy.