Go to any major Broadway website. The best ones include Playbill.com, Theatermania.com and BroadwayWorld.com (see References below for links).
Find the link to discounted tickets on the site's homepage. On a site like Playbill.com, this is called the Playbill Club.
Fill out the short form and register. You will most likely create a user name and password so you can sign in anytime to see the discounts. You can also choose to receive discounted offers by email.
Sign into the discount club and see which show sparks your interest.
At the bottom of each discount offer, there are usually three options for ordering tickets:
ONLINE: Most sites have links to websites like Telecharge.com or Ticketmaster.com where you can submit a discount code and order the tickets right from your computer.
PHONE: You can call the box office and mention the discount code while you are ordering the tickets.
IN PERSON: Print the discount offer and bring it to the box office of the theater and they will give you the discounted rate.
Given the proximity of your ticket purchase, your tickets will either be mailed to you or you can pick them up at the box office.