Welcome aboard cheybay. I hate this for you. I get so frustrated with pet stores because they really have no clue or care about the animals they are selling. If they had males and females in the same cage, they were selling them cheaply as feeder rats, so it wouldn't matter if a female were pregnant or not, and of course they wouldn't have enough basic knowledge to know if they are looking at males or females when they pick them out for pets. I bet money that every single female in that feeder bin who is of breeding age is pregnant probably.
If your female is big and pear shaped, and her nipples are showing, assuming she's pregnant, you've got a week or less until she gives birth.
I am assuming you wanted these little guys for pets and not for breeding purposes so what you'll need to do first is separate your male and female immediately. A female rat will go into heat as soon as she gives birth, so if she has access to a male, she will immediately become pregnant with a second litter. Also, even if a female doesn't have a litter, she goes into heat every 5 days or so, so you'll want to keep her separate anyway.
It is possible and common neuter and/or spay your rats as pets. After that you can keep your male and female together just fine with no risk of pregnancy. Since these are feeder bin rats with no care to their health and genetics, spaying and neutering is advisable anyway. Neutering can help with hormonal aggression in poorly bred males, and spaying can all but eliminate later mammary tumors in your little female. I just lost a favorite female to massive mammary tumors, so I am a cautionary tale for that. It's also a good idea to line up a rat knowledgeable veterinarian asap in case something goes wrong with the pregnancy/delivery and she needs emergency help.
Put your female in a birthing cage asap (flat bottom glass tank or plastic bin to prevent the little pinkies from falling through or getting caught in wire).
You can make your own whelping bin cheaply with zip ties, hardware cloth, and a plastic storage bin. Here's an example of what it look like: http://www.spoiledratten.com/binpics.html
Put fresh clean soft bedding in and don't change it again until the babies are older so you don't stress out mama.
Offer your female a healthy diet with extra protein. She is likely malnourished from her previous life. A hard boiled egg in addition to healthy lab block and the usual veggies.
When the babies come, if she is calm and relaxed you can handle them every day to make them well socialized to people. If mama is fearful and nippy, leave her alone or only handle them when she is out of the cage or distracted while eating.
Here's a few links to help you out:
Signs of Pregnancy in a Rat:
Frequently asked questions from new rat owners about rat breeding: http://www.worldofrats.com/ROUSBreedingFAQMain.html
Rat development from birth to weaning: http://ratgrowth.homestead.com/
Rat Diet(including foods to avoid, just scroll down to see the list):
Proper Rat Housing:
List of safe (and not safe) bedding for rats: