Feelings for my best friend...

Well the tittle says it all... I fell for my best friend and I just don't know what to do about it. He is seriously one of my closest guy friends and I would hate to ruin what I have with him. It's just every time I'm with him I'm in the best of moods and he just makes me so happy. I don't know what to do I am in a real situation and I need some help!