Remove the racks and wash them. Most racks come right out of the dishwasher. Pull them out and look at their track. At some point their should be a dip so that if you slide the rack to that point and lift up the rack will come out. This makes it easier to clean under the racks as well as clean the racks themselves. Because of their size and shape it is usually easier to clean the racks in the bathtub than in the sink. Use dish soap and scrub them down with a rag or brush. Rinse well. You can also do this with the silverware rack.
Remove debris from the arms. Your dishwasher has arms that look sort of like a propeller. Each arm has small holes where the water is pushed out. You will want to make sure each hole is clean and free of any solid matter. If it isn't then pull out debris using a toothpick or a pair of needle nose pliers. Gently clean it out with a soft bristled brush or an old tooth brush.
This step is easiest to do with the racks removed.
Clean around the drain. While you usually don't need to scrub the drain or any parts of the dishwasher it is a good idea to make sure that there isn't any solid debris there. There should be a grating over the drain which will catch debris. Pick any you see up and throw them away to make sure that they aren't being moved around during the wash process (and dirtying your dishes!). You can also use a soft brush or rag to loosen materials up.
This step is easiest to do with the racks removed.
Clean the door. If you check your door there is a seal. This should be cleaned. It can get moldy and catch food. You can use 1/2 cup of bleach or vinegar to 3 cups of hot water and a soft brush or an old tooth brush for cleaning the seal. Simply scrub around it. The under edge of the door is also a place where things can get but that doesn't get washed with the wash cycle. Clean this with the same mixture using your brush or a rag (even paper towel works).
This step can be done with the racks in and even done on a different day if you wish.
Run a cleaning cycle. Leaving your dishwasher empty of dishes and running it will help clean it out. There are several things you can add that can help in this process and choosing one can be up to you and the types of cleaners you want to run as well as any specific problems that your dishwasher may be experiencing.