I started self harming at 11 when I was being bullied at school. There are many forms of self harm but I mainly scratched myself til it bled or cut myself. I honestly cannot remember what was going through my head when I did for the 1st time but I do remember it was a huge release and for just a moment it all went away, all I could think about was the pain and my problems flowing away through the blood.
I'm 23 now and I stopped self harming 3 years ago after I split up from an abusive ex boyfriend. It wasn't easy and sometimes I do think about it when I start to get stressed or down but I realised its not the answer to your problems it's part of the problem itself and it only makes things worse after you've done it not to mention the fact you go to great lengths to hide it.
I think my only advice to someone who is thinking about it is don't do it which believe you me it's easier said than done. Go and talk to someone when I was being bullied which lasted for 5 years at school the thing I was most scared of was if I told someone it would get worse and this was the only way I could cope, that's not true it's best thing you can do, speak to someone before you have the scars to remind yourself of what you did.
I also find positive thinking is an effective tool to cope as well, it sounds silly but whenever I feel down or upset I write a list of all the good things in my life and all the bad things too. Most of the time the good outweighs the bad.