Cages for Rats
A large wire cage is best, especially one with horizontal bars that allow the rat to climb on the sides if they wish. A tall cage with ramps and platforms is ideal for providing room for multiple rats. As minimum, a cage with 12 by 24 inches (2 square feet) of floor space is okay for two smaller rats, as long as the cage is tall and you provide shelves and/or hammocks for extra space. Larger is always better. Large aquariums are okay, but do not provide good ventilation (and must be cleaned more often).
Avoid cages with wire flooring as spending time on wire flooring has been linked to bumblefoot. Many cages have wire balconies and shelves, which are not ideal. However, you can modify wire balconies by covering them with a thin sheet of wood or other solid material (fix to the cage with wire ties). Also look for wire that is a fine grid (1/2 inch by 1/2 inch maximum). Your best bet is to look for cages that have plastic or wood shelving, or you can modify cages using melamine covered boards to make your own shelves (easy to clean). Many enterprising rat owners have built their owne large cages. See the Dapper Rat's Grotto and their pages of other ideas for inspiration.
For bedding, avoid cedar and pine wood shavings (see "The Problem with Cedar and Pine Shavings" for more information), but aspen (or other hardwood) shavings are fine. There are many other good pet bedding and litter options available these days that are very absorbent, not dusty and safe for small pets. Some are pelleted so might not be all that comfortable for rats to play and sleep in, so some people use the pelleted products (which are usually very absorbent) under a layer of softer loose bedding. For a sampling of the newer alternative pet bedding products, see "Top Ten Alternatives to Cedar and Pine."
You will also want to provide some nesting material which the rats can shred and use to line their nest box - paper (no ink), tissues, or paper towels work well.
Usually, the rats will chose a bathroom location in one area of the cage. Heavily soiled litter should be scooped out daily, and more litter added if needed. The whole cage should be cleaned and new liter and bedding provided once a week or so.
A nest box should be provided - and can be store-bought or home made. A cardboard box makes a perfectly acceptable nest box, although it may need to be replaced often. Other possibilities include a flowerpot or jar turned on its side, or a section of PVC drain pipe (perhaps cover one end). Store box boxes are good too, but keep in mind that wooden ones can be hard to clean if they get urine on them, and the plastic ones might get chewed up fairly quickly.
Toys and Accessories
Rats love to climb, and will make good use of ladders, ropes, hammocks, tunnels, and platforms. Toys should be provided as well - blocks of wood for chewing, cardboard tubes, and toys designed for ferrets or parrots are good choices. Look for rope and wood toys as many plastic toys can't stand up to chewing by a determined rat. Simple items like large cardboard mailing tubes, crumpled paper, paper bags, and cardboard boxes can also make wonderful toys. Remember that rats are very intelligent and need to be challenged, so rotate the toys on a regular basis so the rats do not become bored. For more ideas, see "Homemade Rat Toys" and "Top Rat Toys Online".
Some rats like to run on exercise wheels (and some will never try!), but the wire type commonly found in pet stores isn't very safe for rats as their feet or tail can get caught in the rungs or the supports on which the wheel is suspended. A solid surface wheel is preferred, for example something like the Wodent Wheel.
Heavy ceramic food dishes are probably easiest to use as they are sturdy, don't tip over too easily, and they are easy to clean. A water bottle with a sipper tube can be used for water. Make sure a supply of fresh clean water is always available.
Where to put the Cage
The cage should ideally be placed in a relatively quiet location but still near the social activity in the home. Rats are nocturnal so should be located where it is fairly quiet during the day. Placing the cage on a table or stand will help the rats feel more secure. The cage should not be placed in direct sunlight or in drafty locations. Limit access to the cage by other household pets, as a rat will understandably feel threatened by a cat or dog hovering outside the cage.