If you have decided to get a pet rat I've outlined what you will need to properly care for your new pet.
Choosing a Rat:
When choosing a rat you'll want to pick one with zero signs of upper respiratory disease, that means avoid rats with drainage around the eyes and nose, noisy breathing, and odor from the mouth or ears.
Make sure the rat you choose has nice thick fur without hair loss and scabs on the skin, that could indicate parasites. Also look at the temperament when choosing a new rat. Avoid rats that are skittish or aggressive. Take time to handle the rats before deciding.
Male rats are generally larger and more laid back and females tend to be smaller and more active. If you get two rats they'll make great companions for each other but get two of the same sex other wise you may end up with babies in 19 - 23 days, and to give you a fair warning a litter can have as many as 6 - 12 babies. Rats are very difficult to properly sex when they are young, I wouldn't put a whole lot of faith in the pet store.
Cage:
You will obviously need a cage for your rat to live in. A small rodent cage with an exercise wheel works fine but it needs to be a minimum of 12" x 24" (15 gallons). Rats like to climb so add some ladders but avoid the tubes you might see in some cages, they are hard to clean. Rats do like to burrow so add some hiding places and bedding. I would avoid cedar or pine bedding; rats can have an allergic reaction to it and it can carry mites. The cage should be cleaned as needed but a minimum of once a week is best.
Food:
Rats are pretty easy when it comes to their diet because they can eat what we eat. There are rodent pellets available but if that is all they get they can become vitamin deficient - they tend to pick out what they like therefore their diet becomes incomplete if they are only fed the rodent pellets.
There are some foods to avoid due to toxicity, those include:
chocolate,
raw beans,
carbonated drinks,
raw sweet potato,
blue cheese,
raw red cabbage,
raw brussel sprouts,
raw artichokes,
orange juice,
green bananas,
green potato skin and eyes,
rhubarb,
dried corn,
cucumber,
radishes,
spinach,
collards,
turnip greens,
beets,
almonds and,
Swiss chard
Rats do like treats, just like all pets, some good ones include:
Rice Krispies, Cheerios, Total, strawberries, grapes, watermelon, apple, carrots, peanuts - unsalted, if you buy peanuts in the shell it's neat to watch them eat them. Rats are intelligent, you can use the treats to teach them tricks.
Water:
Rats need water, of course, use a water bottle attached to the cage. You should give fresh water daily. Rats also like to wash themselves and their food so adding a small bowl of water in the cage might make them happy.
F.Y.I.:
Rats like baths, just like a dog. They also need to have their nails trimmed - you can learn to do this yourself or take it to a veterinary clinic and ask them to do it.
Rats should be handled a lot for socialization purposes. A new pet rat might urinate and defecate on you at first but this is due to nerves and will stop when the rat gets more used to you and to being handled.
A rats teeth are consistently growing so it will need special chews to help keep them ground down. You can get these at a pet store.
The sad truth is that these special pets don't live nearly long enough. Their life expectancy is about 2 - 3 years. Rats can suffer from obesity, mammary tumors, and chronic respiratory disease. Don't think you can't take your pet to a veterinarian for care, many doctors see pet rats on a regular basis and can treat many of these ailments.