Congrats for coming out to everyone who reads this post. First step accomplished
I came out as gay in my younger years, so understand a little of your apprehension. You've obviously chosen this friend for a reason, could you spell out to them the importance of keeping it a secret until you're ready to tell others (if you're ever ready).
In my experience, many people knew about 'me' before I told them anyway…and they didn't care. You'll be surprised how many signs you give off without realising it.