Previews is a monthly publication printed by Diamond Comics. It has all the comic titles and other related pop culture items that will be available in stores in the next 2-3 months.
You just need to read through the Previews catalog and select what comics you would like to receive and let your local comic shop know. If you have a subscription, those items will be ordered for you and placed in your subscription folder/box.
Most comic stores base the amount they purchase on their subscriptions. They order enough to cover their subscriptions plus some additional copies to place on the shelf.
Since comics are ordered 2-3 months in advance, if a lot of people add a title to their subscription after the order is placed it may use up all the extra copies that were ordered for the shelf. Subscriptions are always filled first so there may not be enough for the shelf. This is why it is important to check Previews catalog each month so you get your order in on time. There is usually a copy of Previews at your local comic shop that you can look through or you can add it to your subscription and have your very own copy to take home and peruse.