After you leave the customs area, you go straight through the main hall exit out the door and the taxi stand is on the right. The fare including the 50 baht airport fee and the tolls should be about 400 baht. Don't listen to the touts in the main hall. When you get outside say Taxi meter (or someone will ask you) make sure it is a metered taxi.
You want Mo Chit Song (terminal two)(?????????) as Mo Chit Neung (terminal one) is for the local buses. Also be aware that there is not a lot of English spoken at the terminal. The first floor has buses to the North (Chiang Mai, Chiang Rai etc...) and the third floor has buses to the Northeast Udon, Nong Khai, Sakon Nakhorn, Mukdahan etc..).
There is a information desk in the center of the room on both floors but the touts at the bus station will direct you to the right counter. You probably want a bus gow, gow,gow (999) or VIP as they are the better class of bus and air conditioned.