I told one of my best friends that I would never talk to him again if he told my parents I was secretly going to go smoke a cigarette in the woods.
You have had this best friend for a long time. Why are you so upset over his actions? Why are you so worried about him telling your parents about your smoking?
Because I just don't feel safe telling my parents about those things.
While it is understandable that you are upset at your friend for wanting to tell your parents about your smoking, you have to realize that he is trying to look out for you and to stop you from making poor choices with your health. Smoking tobacco while you are a minor is illegal and wrong and especially so since it is so habit forming. Instead of being upset at your friend and threatening to cut him off, you should stop smoking and take better care of your health.
Ugh, fine, but I still don't want my friend to betray me.
I hope you will not smoke tobacco and you can ask your friend to respect your privacy.