Ensure you have sufficient working capital. Diamond is the major distributor, and it will be almost impossible to run a comic shop without working with them, as they have exclusive relationships with most of the major publishers. Diamond recently raised their minimum order amount to $425 before any discounts, so make sure you can afford to do business with them each month.
Consider alternative distributors. For example, Haven Distributors specializes in indie comics and has a low minimum order to qualify for wholesale prices.
Apply for a Diamond account. You may need to wait up to two weeks after applying to be able to order through their system. Once accepted, you will be assigned a customer account number.
Fill out the order form. When dealing with Diamond, you can order via the Previews order form, the Diamond website, or the Previews Program for Windows.
Wait for your items to arrive. Wednesday is new comics day, so be prepared for an increase in traffic to your store each Wednesday.