how do I know if I should go to a doctor after self-harm?

No matter how minor or major, you should always actively seek help whether you are currently or considering self harm. Seeing a doctor is the second step to recovery, I think the first step is too be aware of yourself. I have a friend who was thinking of self harm and we supported her as much as we could and when she told us she was going to seek help, we all supported her and saw her become much happier. She is now starting her first year at university studying Psychology, everything gets better with time including yourself.