I was extremely nervous when I came out to my best friend. We've known each other for about 8 years and he didn't know about me this whole time. One day when I was really depressed about a break up of my partners of 3 years, I decided to blatantly tell him what I've been doing this whole time.
To my surprise, he took it very well and we're closer than before ( I guess I should have mentioned that he's straight). Anyway, every now and then he'll give me crap about something (jokingly) but I just give it back to him.
My advice is don't stress too much because if they're truly your best friend, they honestly wont care or see you any different.