Break up with him in person. No matter how tempting it may be to send him a break-up email, be mature and take him somewhere where the two of you can be alone.
Explain to him the reasons why you feel that the two of you shouldn't be together anymore. If he did something hurtful to warrant the break-up, then feel free to handle the explanation however you see fit. On the contrary, if the two of you had a fun, meaningful relationship and you're just ready to move on and see other people, be as sensitive and kind as possible.
Discuss how you will handle social situations after the break-up if you share the same friends. Ask him if he'll be alright hanging out at the same places as you immediately after the break up. If not, figure out a way to nicely avoid each other until some time passes.
Make it clear that this is a permanent break-up. Be direct with him and tell the truth even though it's difficult. This will ultimately help both of you to get over the relationship and move on.
Allow yourself time to heal. Even though you were the one who broke up with him, it may still be hard for you to accept the end of your relationship. Give yourself time to cry it out and eventually move on.