Don't worry about it boy. Masturbation is as natural as breathing for a young man like yourself. Don't be ashamed of it or imbarrassed around your dad, he did it at your age and I'll bet he still does it from time to time these days. You're doing right to try to keep it private, but it's nothing to be ashamed of. All people, men and women masturbate now and again. (Besides, it's fun and free. You got that toy for nothing.)
Your 15! Yes its normal but the porn may end up a problem. I agree with margypops, the computer should never be somewhere parents can't see it, but we aren't talking to the parents here. You shouldn't feel embarassed, its normal, but i would suggest doing this in your own privacy where no one can just walk in. Your father didnt say anything because he knows its normal. I am sure many young men have been caught by their mother or father. I wouldn't worry about unless there are other issues.
It is part of growing up we adults call it Hormones going wild, I think your parents should have the PC where everyone can see it, the Porn is not a good idea in a place where people live ,however your Dad must have considered it okay to let you stay doing that. You have your own answer you will have to bear the Embarressment and the lecture.