There are many tour groups for you to choose from that charge different prices, but the one of the best known and most popular ways to visit the DMZ is with the United Service Organizations (USO).
The USO DMZ tour can be booked by visiting the Koridoor website at http://www.koridoor.co.kr/bbs/board.php?bo_table=travelInfo&wr_id=221. Koridoor is an official affiliate of the USO. Click on Schedule & Bookings at the top left when you are ready to choose a date.
Tours can sell out quickly on weekends and in the summer, so try to book as far in advance as possible. The tour schedule, including dates and times, can be viewed at http://www.koridoor.co.kr/bbs/board.php?bo_table=schedule.