I'm even more old-fashioned.
I have some serious issues with you having this serious a relationship with a 17 year old. I think you need to have a conversation with your Mom, but there are two topics you should discuss with your Mom before you talk about birth control:
1. Your relationship with this guy... the age differences (they are important at your age), what exactly the relationship is and where it might go, etc.
2. The "birds and bees" - not so much in terms of a "how to" but in terms of how sex fits into a relationship and how having it/not having it affect the relationship.
You won't have to dig too far into these forums to find people who regret becoming sexually active too early. Read the thread "Compaired." There's another thread (can't remember the title) that will show the panic that happens when you think your pregnant in spite of using birth control.
Listen to the advice and learn from other's experience.
Wally