You need to get two of the same sex or be prepared to have two cages and the money to neuter the male.
PLEASE do your research before purchasing rats. I have five females, although I did not make my first purchase until I was more than informed about them. Make sure you have the proper facilities (read: cages) to house such small rats and make sure you are either able to purchase a larger cage as they grow (rats needs at least 2ft of cubic space each) or have the funds to purchase a large enough cage initially. Kudos to you for getting two - they are extremely social and just aren't happy by themselves. I recommend hanging around a few different rat forums too to make sure you are up to ownership.