Try and clean the drain fist. Remove the drain port cover and tread a slim rod or auger down the drain the try and clear the drain/trap. Remove the trap and clean that also. It is located under the dishwasher. You might have to move the dishwasher in order to get to it. Also check the strainer located inside the dishwasher. Sometimes that also becomes clogged. You can lift out the strainer and clean out the ports in the sink. The strainer may be metal or plastic and usually is in two parts. To avoid a clogged strainer, you should rinse off your dishes before you put them in the dishwasher unless you have a dishwasher designed with a food disposer. If you have cleaned everything and the dishwasher is still backing up its time to call the plumber, something else is causing the problem.