A number of things could be causing standing water in your dishwasher.
First of all, make sure that the door is completely closed.
Press the “cancel” button (sometimes twice, depending on model) to manually drain water left behind because the dishwasher cycle was interrupted (i.e. kids/roommates). The dishwasher will turn off after a couple minutes.
1. If you have a disposer, run it, because your drain system could be blocked.
2. If you have an air gap, it could be clogged. Consult your particular brand for instructions on how to clear that, but mostly it’s just taking off the chrome cap, unscrewing the plastic cap and cleaning out whatever’s ailing it. (Here’s a video by GE that’s kind of fun.)
3. Make sure you’ve got drain hose in right dishwasher area codes. The drain hose should be looped to touch the underside of the counter and connect to the sink drain or disposer. The hose should not exceed a foot in length. Also, check for any kinks.
4. If you’re still under water, call a plumber (or go cheap like me). Your sink drain could be clogged, which prevents your dishwasher from pumping out water.