Why need to tell them anything? I don't understand this whole "coming out" thing where there's a ritual of letting people know your sexuality unless you're straight then it somehow doesn't apply. I never assume someone's sexuality unless told and even then don't care one way or the other. The trick is just don't care at all. If some of your friends give you any kind of a hard time over it, fuck them and just find new friends to replace them. There's plenty of people in the world and most aren't assholes. If someone "doesn't believe you", who gives a shit... it doesn't affect their life in any way. And if they give you shit about "not believing you" then fuck them and replace them too. Any good, mature friend that you'd actually want in your life won't care in the slightest and will give you the acknowledgent of "yup totally cool" and nothing changes. And if you meet someone who's also gay then there's never a better time that you should be more certain of not needing to feel embarassed. So just practice not giving a fuck little by little.

For the record, I'm straight and never had any of my gay friends do the whole classic "coming out" thing in front of me. I'm not even sure what the correct response would be other than "uhh alright cool it's not important to me, nothing's changed"... honestly I may be upset for a brief second that he/she feels that I'd reject them as a friend after already having been friends. Every gay friend of mine has "come out" by just simply not trying to hide it the first time anything related to sexuality came up in a normal, relevant conversation. This is how it should be in my opinion... just tell people as soon as it becomes relevant in some way - there's no need to go declaring it on a whim to "get it over with". There was one time long ago when I incorrectly assumed a newer friend was heterosexual on our way to a party and he just immediately said "Nope - I prefer the boys so let's re-strategize" and neither of us gave it a second thought.

PS - 23 isn't old