Mattresses are typically sold as a set with a matching foundation, a mostly rigid support system that is designed to go directly below the mattress. There are a number of types of foundations, and it is always recommended to purchase a mattress and foundation together. Mattress foundations help your mattress to live longer and stay comfortable, by absorbing most of the stress and weight placed on your bed.
Depending on how many inches thick your desired mattress is, you may want to consider weather to get a standard or low-profile foundation. So, you won't need a ladder to get into bed ;)
Standard Foundation: 9" Thick
Low-Profile: 4.5" Thick