Housing a rat is not as simple as buying a cage and shoving them in there, then leaving them in a corner somewhere in your house. Rats are highly sensitive to their environment, and living in a boring, sterile home will make them stressed and unhappy. That's not to say you should randomly pack their cage full of random exciting things; rats require specific features installed in their cage so they can be happy.
If possible, find a larger cage for your pet rat, at least 2 square feet of floor space assuming the cage is tall and there are hammocks and/or shelves for extra room. The larger the better, but everyone has space considerations in their home.... just do what you can! Rats enjoy climbing, so ideally the cage should have horizontal bars with ramps and platforms to climb on.
In terms of cage location, try to place it in a relatively quiet place in your home, but still near enough to your family's activity so your rat feels included. Being a nocturnal animal, rats will need relative quiet specifically during the day. Avoid drafty areas or those with direct sunlight, and of course keep your other pets away if possible. Your rat will definitely feel threatened by your dog or cat hanging around nearby!
Fortunately it is quite easy to fulfill your rat's nutritional requirements through readily available pellets or blocks. These are specifically formulated to provide the nutrients your rat needs to stay healthy and happy, but you can go one step better by looking for low fat / caloric content, as well as soy meal over corn as a major ingredient.
Although these pellets will provide your rat with the basics of their dietary requirements, you should also provide some variety with fruits and vegetables, wholegrain pasta or bread, cheese and nuts. You can provide these as treats so your rat can bond with you, or purely so they don't get bored with pellets!