i understand that because it's a religious thing it's a sensitive matter and as much as everyone else is telling you that it's your hair and you should do what you want it's not as simple as that as you still have respect for your dad and his religion. I know a lot of sikhs who do cut their hair. do you have any relatives who do? any family friends your dad respect that he would maybe listen to? you can do your own research and find scholars who do accept cutting hair but to a manageable level, this is because in today's society extremely long hair can become a burden in jobs and in hygiene. do your research. if that fails then I think you need to sit down with your dad and explain to him that although you respect his religion you have your own ideas as well. if you practice sikhism yourself point out all the positive ideas and concepts that you have learnt, understood and embraced. explain how by wanting to cut your hair, you feel it's a trivial thing compared to all the other requirements sikhism has such as wearing your bracelet, being a good person going to temple, refraining from alcohol etc. explain that by cutting your hair you are not trying to distance yourself from him or his faith. I think if you have an issue with sikhism as a whole it's best if you didn't bring that up just yet, wait till your 18 when you've done your research and are sure that this faith isn't for you. if you tell him that you gain nothing from sikhism it will hurt him and his faith is so close to his heart and I'm sure you understand that although other people who haven't been brought up by religious parents may not. when you're in a position where you can financially and practically support yourself you can take that risk. also when speaking to your dad, he probably will be quite angry and upset and first and you must be gentle and show him that you respect him and be mature. give it some time and speak to him again, show him you are mature and this is something you believe in as much as he believes in his ideas. be persistent but don't lose your temper or sulk. show him you are serious and hopefully in time he will understand. good luck!