Talking to parents about sex...

Ok so I am 15 (I know I'm going) and am in a really stable and happy relationship, my mental health is at its best that it has ever been at for the last year, and I feel really strongly about my partner of two years. So I feel fully ready to deepen that bond... but I don't know how to tell my mum. Or my dad. Should I tell both my parents? Or just one? How should I tell them?