Do best friends think sexual things about each other?

First of all it's a bit funny that your profile age says you're 25-29 but you say you're only 17.

Regardless. Best friends certainly think sexual things about each other - especially if one's a guy and one's a girl. It is a simple truth that guy and girls can never be 100% friends without feeling at least 0. 001% sexual affection of one another (unless homosexual).

It is wrong to tell her if she is simply your friend. Because why are you telling her for? Ask yourself that question, and listen to the answer that your heart says.