The minivans to Hua Hin and Cha-Am (and other destinations) leave from the soi (lane) next to the Century Movie Plaza. The fare is 180 baht per person and they leave frequently (I think every half hour).

Here's what the van station looks like.

Here's the layout of Victory Monument.

If you REALLY want to save some money, take a minivan TO Victory Monument. They cost only 40 baht from the airport transport centre and come into the soi next to the Fashion Mall. To get to the soi where the vans to Cha-Am go, you'd have to go up on the walkway, so this is not recommended for people with heavy luggage.