The first time self-harming isn't the best feeling. I do think though that you seem aware of it, and that you can prevent this from occuring more. Definitely check out the good alternatives list to self-harming (I'm sure you can find something that will help you out). Something that I feel helps is to take deep breathes, and practice breathing in with your stomach. It does help you feel more relaxed, which should help with the need to self-harm.
I agree with Jenn - you should talk to people about this. It can only help how you are feeling so that you don't feel that you are hiding anything or alone. Perhaps you could try seeing a counselor? I think it would help in finding coping methods to help deal with things such as your dad. I would advise trying to avoid your dad if you feel that you can't talk to him, or he can't change his attitude towards you.
I hope that everything does get better. PM
me if you do want you talk :]