there is no easy way to tell your partner its over, so do it nicely, and make sure you have a reason that your breaking up with him/her. do it in person, make sure of that, and make sure you do it when you are alone, not with 10 other people surrounding you. that is a mistake many people including i, have made.
always try to make sure you are friends after the relationship, so if that person you went out with relied on you alot, has sometime to make some new friends.
hope this was the sort of relationship you were talking about, and hope that you take my advice and it goes well.