If you litter train your rats, it makes it easy to do a quick exchange of the litter tray every day or two. It's a good idea to set a regular schedule for fully cleaning out your rat cage. How often you will need to clean out depends on the number of rats you have, how large the cage base is and what litter you use, but I would say a fortnight is the longest you would want to leave the cage.
Change out any hammocks that have been peed on, take the wheel out (if you have one) and any toys to rinse, empty out the litter and wipe out the base. Then spend some happy time rearranging the cage. I like to let the rats 'help' while I'm doing this, but if you'd rather not have their help you can pop them into their carrier or a small cage while you clean their cage.
Avoid using strong smelling cleansers, as these will prompt the rats to attempt to cover the smell with their own.
Water is usually best in a pet water bottle attached to the cage rather than in a bowl, although bowls are sometimes better for older rats who can't sit up to drink. Water bottles should be refilled regularly, at least every second day, even if they haven't been emptied. I prefer to hang two bottles on each cage in case the spout fails on one of them. I use my usual fluoridated tap water for my rats, and don't believe it's caused them any problems over the years.
Rats have sharp little nails. On the one hand it's useful, because it means they can climb up you and hang on well. On the other, it means they will leave scratches all over any bare skin. There are various ideas that come up now and again, such as using rocks or bricks under the water bottle or using perches and shelves made from rough materials to wear their nails down. My experience is that unless you have one of the wonderful rats who actually trims their own nails, you will need to trim their nails on occasion. If you're going to enter your rats in a show, they will need their nails trimming. At many shows you can prearrange for someone to show you how to do this, but the essence of it is as follows:
You will need either a pair of animal nail scissors, which have a notch in the blade for the nail to sit into, or a pair of nail clippers for people. My preference is for the nail clippers, but it's an individual thing. It's also useful to have some bloodstop powder or a small amount of cornflour on hand in case the nail gets cut a little too short. Get someone to help you. It is possible to do this alone, but I would save the attempt until your rat is more used to having his or her nails clipped. The holder needs to hold the rat belly outwards but against their chest, with the paws sticking outwards. Hold the rat's foot and look for the white area at the end. With most rats you can see through the nail to where the blood supply stops and where it's safe to clip. If the nail is curled round very close to the toe, put your own nail down the gap to shield it as you cut. Clip squarely across the tip of the nail, and move along as quickly as you can. Take care, because the rats will push their feet as well as pulling, and may well get their whiskers in the way if you do their front paws as well.
Rats very rarely need to be bathed; it's usually the cage that smells rather than the rat. If there's a smell that you can't clear by cleaning the cage and changing the hammocks, check the bars and the wall behind the cage before you start rat bathing.
If you feel you must bathe your rat, use a baby shampoo or pet shampoo. Fill the sink before you bring the rat into the room, and have a towel to hand in advance. If possible, sit down with the towel on your lap and then lower your rat into the water. Retrieve the rat from the top of your head, and work in a very small amount of shampoo. Rinse your rat in the water, retrieve them from behind your neck, and wrap them in the towel to dry and comfort them.
You will now need to grovel and offer treats to your rat before they will deign to notice you again.