Go to your reddit preferences and choose the deactivate tab at the top of the screen. Follow the on-screen instructions deactivate your account. You can also click this link. You will need to know your account password in order deactivate your account. If you do not know it, and haven't stored it anywhere, there is nothing you can do. The Admins will also not be able to help you.
Please note that if your account is deactivated, nobody can ever use that username again. Usernames cannot be recycled and accounts cannot be reopened.