Go to the website of the theater you plan to attend. Click on the 'purchase tickets' tab. Occasionally this will lead you to another ticket-issuing website, such as Ticketmaster. These companies provide online ticket purchasing for any size theater in exchange for a percentage of the ticket cost. You will need to use a credit or debit card.
Preview your ticket purchase to make sure it has the correct date and price. Some theaters will honor a ticket that was purchased for the wrong date, as long as the seats are not full. But you have no guarantee that you will be admitted.
Print your tickets. Depending on how you bought them, you can print the tickets directly from the website or wait for a an email with a ticket confirmation and a link to a printing program. Bring the printed tickets, and the credit card you used to buy them, to the theater on the night of the show.