How do you tell a boy that you don't want to be with him anymore?

You tell him, straight out, preferably to his face, but on the phone is okay. You have to decide what's better. If you do it over the phone, it's better for you to initiate the call than to wait for him to call and spring it on him. You just have to face the fact that he will be upset, perhaps very upset. But you can't pretend you have feelings for him just to spare his feelings. Tell him without beating around the bush, but don't get nasty or accusatory and be respectful of his feelings. Make a clean break and move on. After the breakup, try to reamin civil, assuming he does so, too, but "remaining friends" may not be a good idea. It usually leads to more trouble in the long run and may make it harder for him to get over you.