There's been good advices given already, so I will mostly stress on the most important points and add some links.
1) Do not keep a rat alone. They definitely need at least one companion of the same sex. As has been mentionned, rats are lazier and bigger while females are smaller and more active.
2) Do keep in mind their different needs:
The environment:
a) a good sized cage. You can use the rat cage calculator (http://www.fancy-rats.co.uk/information/guides/cagecalculator/) to know how many adult rats you can comfortably house in the cage you're thinking about.
b) do not house them in tanks, it has really really poor ventilation and they are then more prone to respiratory diseases.
c) although many people keep their rats in ferret or chinchilla cages, those cage usually have a bar spacing too big for young rats, who can just go between the bars. So if you keep young ones, keep that in mind.
d) rats love to climb, so prefer a tall cage with platforms to a wide and low one.
e) choose your bedding carefully. Stay FAR AWAY from pine or cedar beddings, they are the worst you could ever use. I prefer hemp, but it can be hard to come by, then I go for aspen. Many will suggest trying carefresh, but I find it too dusty. Corncob also has the bad tendency to mold when too moist.
Nutrition:
a) Rats like diversity, but they need a balanced diet. I personally prefer to feed them a rat/mouse mix plus specially formulated pellets that offer a complete diet. I refill their bowl when they have pretty much ate all but the alfalfa bits in the mix and ate all their pellets.
b) Rats love treats. You can buy commercial treats, like yogurt drops, which they love, but just don't abuse those, especially for the males who will get fatter. They enjoy fresh veggies: carrots, brocoli (good way to not waste the stems), peas, hard-boiled eggs, yogurt, soy milk. They can eat basically all you eat, but don't give them junk food. Also avoid anything that's sticky in big quantity, like peanut butter, as they could choke on it (instead, spread a very thin layer on a small piece of toasted bread).
Health:
a) never forget that rats are rodents, and that their teeth constantly grow. They need to have access to something to chew, and no their cage bars do not count!
b) females are more prone to tumors than males, so you might want to keep that in mind
c) all rats carry a bacteria in their respiratory tracts. Most are healthy carriers, but in the case of a bad bedding that irritates them, the bacteria can use the opportunity to make the rat sick.
Other stuff that you will need:
a) a hiding place where they will feel safe.
b) toys, because rats love to play. Bird toys work well, some cat and dog toys as well. I personally love to hang stuff to the bars of the cage or toss a cat ball in the bottom. Avoid any cat toy that contains catnip though!
c) hammocks, the rats love to sleep in hammocks
Other random tibbits:
a) Rats can be potty trained, they can learn to potty in just one particular spot.
b) you can teach them tricks, and that includes comming to you when you call them, very useful for play times outside the cage.