I think I'm in love with my best friend
What makes you think you are in love with them?
I'm not sure, maybe it's just because I'm young.
If you are in love with your best friend then you should ask them out on a date. It's a risk but if you feel that they feel the same way then you can make your relationship official.
I just don't want to ruin the relationship we have.
I understand your worry; it can be scary to change a relationship dynamic. If you aren't sure of your friend's feelings, you should take things slowly and try to see if they feel the same. You can tell them how you feel, just assure them you are happy to remain friends if they don't see you as a romantic partner.