As a person who's been around a wide range of pets from the cradle up, I can tell you for a fact: rats are really cool pets to have. Contrary to popular prejudice, rats can be very sweet, very tame, very affectionate creatures if raised and handled regualrly by humans. They're very friendly and will lick you and wrestle with your hand like a miniature dog.
Rats are the ideal pet if you have allergies. They won't shed everywhere like a cat or jump all over you and cover you with mud like a dog. They only urinate outside of their cage if nervous and are, therefore, easy to clean up after. No potty training is needed. Simply place the damp newspaper in one corner of the cage, and your rat will proceed to use that corner as her or his own restroom.
Rats are also dirt-cheap to care for. For one thing, they eat anything. You can feed them cheap foods from bird seed to cereal and they will be happy and healthy. Your rat needs a toy? No problem. A shoe box will keep it busy for hours as will toilet paper rolls, cheap dog chews, and old socks you can dangle from the top of the cage.
If handled and held everyday, a rat will soon learn to sit on your stomach, do tricks for treats, and love and trust you completely. This isn't always true, however, concerning an adult rat. An adult rat that hasn't been handled and played with regularly may be hostle or frightened and so it is always best to purchase a rat as a baby.
Some advice for parents: purchasing such a small pet as a rat and teaching your son or daughter how to care for it is an ideal way of teaching kindness and responsiblity. It worked with me, and my mother wasn't even trying for that outcome!
So you wanna buy a rat? I've convinced you then.
It's always ideal to look for a young rat. Now keep in mind that rats are very sociable creatures and one rat alone will be very lonely unless its owner can spare a lot of time. So if you find you haven't a lot of time, then consider getting two rats of the same sex and spending as much playtime with both as you can.
So now you're in the pet store and you see all these young rats frolicking and curled up sleeping and nibbling seeds. The first thing you should look for is a rat with clear unfoggy eyes, glossy fur, no bald patches, and no blood on the nose. A rat with blood discharge from the nasal passages will have a respiratory infection and will probably not live very long. Also pick the rat up and feel for odd lumps. A rat with a lump will very likely die from cancer. I can remember so many sad times when we purchased a rat and not long after we brought it home it died of cancer.
Finally, look for two rats who seem to get along. Like people, some rats just don't like each other and two male rats will fight for dominance of their cage.
Now you've bought your rat and you've brought it home. You're very excited and you can't wait to play with it. You scoop it up and start holding it, right? Wrong. Your rat will want to get a feel of its surroundings, find its toilet and its bowl and its bottle of water and chew toys. Let it get settled the first day, then gather it carefully in your hand and let it sit on your shoulder, hold it, pet it lightly, feed it from your hand so that it learns to trust you. Feed it something like peanut butter so it can't just take the food and run and has to eat from your hand. When your rat realizes that you only want to feed it and play with it, it will love you.
If you purchase to two male rats, it would be ideal to buy a meduim to large cage as male rats get very big and are territorial. Two females will do fine in a medium cage with two stories or even one. Buying the cage for about twenty bucks is the most you will spend on your rat. I promise. Litter can also be purchased in the store or can be substituted with newspaper. Newspaper is less expensive (it isn't expensive at all if you get if from the library). Also, always make sure your rat or rats have enougn water and that their cage is kept in a stress-fress zone. In other words, keep Whiskers and Sparky far away so your rat stays safe from harm and doesn't have to shed fur from stress.
With the basics covered: have fun!