Coming out is one of the most daunting thing that a teen or young adult can do, the idea of telling your family and friends what seems to be a very private secret can be a scary prospect. There are a few things to consider before you bite the bullet, whether the people you are going to tell going to react well is obviously the biggest one. Try to take as objective an eye as possible when considering potential reaction, perhaps try testing the water... drop into conversation some subtle reference to a current gay star, or character, from a TV series or band, perhaps mention that a friend of yours is gay, and gage their reaction.
Generally people harbour an irrational belief that their parents and/or friends are homophobic when they are in the closet, sometimes this is actually the case (and perhaps it is advisable to not come out until you are no longer reliant on the people in question) but usually it is merely the work of the teen mind (we all like to believe our parents are out to get us).
When it comes around to telling them it can be useful to go through what you would say in your head so that you are absolutely clear before starting. The usual way is to strike up a conversation with a question "What would you think if I told you I were gay/bisexual/trans?", but coming straight out with "I am gay/bisexual/trans" can sometimes be the most straight forward and undeniable approach. After coming out your parents or friends might act unpredictably, do not be immediately concerned about this, it is quite normal for it to come as a shock and therefore, in the heat of the moment, some irrational things may be said (which are often not a true reflection of their feelings on sexuality/gender). A typical response is "You cannot possibly know, you are too young", or "I had a female/male friend who said he was, but just hadn't had a girl/boyfriend (respectively)", I have heard some more bizarre responses as well, just make sure you confirm that you know your mind, and that you are not taking the decision to come out lightly.
Your family and friends might also be worried for your safety, many of them will not know the real risks of homophobia but will probably have heard horror stories about people being hurt or killed because of their sexuality, try to assure them that these attacks are very very rare in modern Britain.
Should you receive an abusive response, especially if it is physical, you should seek help immediately (Gay Youth UK provide a list of switchboards and youth groups where you will be able to speak to someone). If you are physically attacked you should seriously consider calling the police, many of them have specific LGBT representatives and can be contacted via the Report-It scheme. Generally speaking people's experiences with coming out are cathartic (a weight off of the shoulders) not violent, so unless you have reason to believe they are homophobic you should be fine.
Friends are more often than not the easiest people to tell first. Having someone your age that you can talk to can lift a great deal from your mind and knowing someone is always going to be there for you is comforting, and can really help build your confidence when coming out to others.
You might have the feeling that telling a friend that your are gay, lesbian or trans could ruin a great friendship, but in many cases coming out to a close friend can create a stronger bond and bring you closer together. One might say that coming out to your friends will show who your real friends are – the ones who will continue to be there for you and respect you no matter what.
Although friends are more likely to accept your sexuality, it still may be wise to gauge how they may react beforehand, and be certain that you’re telling the right person – someone who you are sure will not let slip to anyone else and will be there for you when you need it most.
After you come out, your parents or friends may want to know more, or may need some help with coming to terms with the various complexities that surround having a gay or trans daughter or son. A great service to support the families and friends of gay and lesbian people are FFLAG who have a support line and a very informative website.