Rat's that are kept in unsanitary conditions are more likely to develop health conditions, and a dirty rat cage is the leading source of foul odors. Clean your rat cage at least once a week, or more often if you have multiple rats housed in a small cage.
Dump and change the litter, rinse off all cage surfaces with a hot water/safe soap mix, and disinfect with a water and bleach solution. Which is one part bleach mixed into 10 parts water. Or, spray surfaces with either vinegar or peroxide and wipe down.
Then wipe down all surfaces and any toys. Wash food bowls and water bottles on the hottest setting in your dishwasher or very hot soapy water to sterilize them at least once a week. Stick to these cleaners unless your veterinarian recommends something else, and always rinse every toy and cage part thoroughly before putting them in with your rat.
An easy to clean cage design will make cleaning easier. The best designs seem to be those that lift up out of their bases for easy dumping and rinsing. But no matter what your design, cleanliness and ventilation are crucial.