The relationship probably started out as a lot of fun and you have had some great times together, but all good things come to an end and most relationships end up with one party dumping the other. This can sometimes be acrimonious, with both people realizing that the fun has gone from the relationship and they are better off apart. But sometimes it is up to one person to break it off with the other, when they have fallen out of love, or met someone new. This is always a difficult thing to do, and hurts both people. But there are ways to make it less painful and to try and make it easier on the other person.
Honest is the best thing you can do here. It is going to hurt either way, so you should make sure you at least give the other person the real reason for the break up. If you have been a naughty boy and have cheated and met someone else, then you have to come clean here and tell them. Do not try and pretend it was something else in the relationship, a problem with them, just come clean and tell the truth. Conversely, if it is something about them, then you have to be honest and tell them. This is the only way they might realize a problem with their personality or self and fix it for future relationships.
Keep It Respectful
Though you are telling home truths and things that will obviously hurt them, it doesn’t mean you have to be nasty. They will likely be mad and you and call you all the names under the sun, but it will pay in the long term to stay respectful to them and try to let them down gently. Even if there is something about them that you have grown to hate and that you feel you have to tell them about, you can always do it in a nice way rather than be nasty or menacing. This is the best way to get them to listen to you and the best way to leave the relationship in some decent form.
Make It Final
It can be very tempting to back out at the last minute when you see how devastated they are, or give them a glimmer of hope of some reconciliation in the future, but in reality, this serves neither of you well. If you really do not want the relationship to continue and you want to end things forever, then make sure they understand there is no hope left in the relationship. Making it final may hurt more in the short term, but in the long run will help you both move on to new things and to allow you both to possibly either remain, or become friends in the future.