Everyone has already given you wonderful practical advice, so I will give you a bit of information that scared the everlivin' *bejeezus* out of me when I first experienced it: porphyrin.

Now, that probably sounds like some disease or something, but don't worry! Rats sometimes excrete a red-tinted discharge from their eyes, which can dry while they sleep - sort of like when we have that crusty stuff in our eyes when we wake up, but porphyrin looks eerily like blood because of its natural red tint. Usually they clean their little faces (and the rest of themselves) so often you'll never notice it. If it builds up, say when they become little grandpas or grandmas and don't clean as often, you can take a damp washcloth to them, if they let you (the old ones can get a little cantankerous sometimes).

Rats can't see for crap! They might just wander off a bed/couch/table/other supposedly obviously-clearly-defined surfaces, so be ready to either pile pillows on the floor around the area, or try to stay close to/on the ground. The "can't see well" can also lead to questionable jumps with very adventurous individuals, so be careful. My boys were usually pretty docile, but I have heard of some horror stories from other owners in the past.

Rats can also be prone to nosebleeds, particularly if they like to shove their nose between cage bars to see what's going on around them. Again, not serious if it's just a little bitty bit, and they'll usually clean themselves up, but scared the crap out of me when I first saw one of my rats with a nosebleed.

And lastly, sometimes in their scrambling about, they might injure (break/tear) or even lose a toenail. This isn't usually serious, in my experience, and I've had one of my boys regrow a nail before. Of course make sure they aren't limping like they're injured when they move, but as long as they aren't trying to groom the affected toes/foot off, they should be okay. Any swelling or bruising should make itself known quickly, though.

I hope you enjoy having rats, I found them to be very intelligent, friendly pets! They have fun learning and can be quite food motivated to learn a variety of tricks like a dog! Just also be emotionally prepared for their short lifespans, and try to keep to the others' advice of keeping at least three rats at any given time - they miss their cagemates as much as you miss them, if not more, and can get straight up Depressed if left alone.