If you don't have a Costco membership, you can purchase things at any of their warehouses and get one membership card. You can also purchase Costco membership cards in any Costco store, at Costco.com, or by calling the store's customer service number.
If you've already joined the program, there are several ways to upgrade your membership status. One of the easiest ways is to purchase a gift card. If you don't want to buy a gift card, you can upgrade your membership status by filling out an application form and mailing it in.