The filter is the first stop. It sits under the lower rack, usually a cylinder you twist a quarter turn. Rinse it in the sink. Labels, olive pits, and popcorn kernels pack the mesh until the pump cannot move water. Put the filter back seated. A filter left out lets that debris go straight into the pump, and the next failure is a jammed impeller.
Follow the drain hose from the pump to the sink. It should climb to the underside of the countertop and then drop. That high loop, or a chrome air gap on the sink deck, stops dirty sink water from siphoning back into the dishwasher. A hose that runs straight down into the disposal will refill the tub after every drain.
If the disposal is new and the dishwasher has never drained, look at the disposal's dishwasher inlet. New units ship with a plastic knockout inside that port. The hose is clamped on, the pump runs, and the water has nowhere to go. Knocking that plug out, with the disposal unplugged, is the whole repair. Fish the plastic piece out of the disposal so it does not jam the shredder ring later.
If the filter is clean, the loop is high, and the pump hums but moves nothing, the drain pump itself is the likely part. Tell the shop whether you hear that hum. Silence during the drain portion of the cycle points at the pump or the control that should power it, which is a different invoice from a clog.
The rest of the dishwasher faults are on the dishwasher page.