Step 1: the filter. In the bottom of the dishwasher (underneath the bottom tray) there is usually a filter that you should be able to remove. Bring it out - there's often gunk on it, and have a wee fish around with your fingers where it came from in case there's food debris in there too. Give it a good rinse, then replace.
Step 2: the whirly arms. If you have a look at them, there are usually a number of holes along the top. They should be clear, but often things get stuck in them - certain seeds, bits of sweet corn, lemon pips. Sometimes they are removable - there's a thing you should be able to turn with your hand where it joins onto the top basket. (The bottom one you can sometimes just remove, sometimes you have to use a cutlery knife to lift up a couple of plastic bits). Run water through them if you are able to remove them. Tip it back and forth. Shake them. If you can hear anything inside, shake, shimmy, tip water through, do whatever you need in order to get rid of it. Reassemble.
Step 3: Dishwashers are magic, but there are certain things that are kryptonite to them. If you have very seedy bread or cereal, try to avoid the seeds going in. If your plates are very dirty, try to get rid of most of the solid bits. And correct stacking procedure can really help - if you can't draw a direct line from the swooshy arms to the dish, it probably won't get clean.
Sorry, essay alert, but I used to do a LOT of cooking for other people, and the dishwasher was the best kitchen appliance. Hope that helps.