As your are 16 you have the right to obtain therapy, your parents will need to know but they have absolutely no right to know what you talk about, your diagnosis or read your notes. It's totally up to you whether you tell them any details or not.
Due to the sensitive nature you also don't have to tell your GP the details or specifics, all you need to tell them is that you have intrusive thoughts revolving around causing harm and that you would like help. You don't even have to tell them that, it's up to you what you tell your GP but you will have to tell him/her something in order to get a referral.