There are 4 stations around Bangkok : Ekkamai , Morchit ,Sai Tai Mai and Suvarnabhumi airport stations  

EKKAMAI STATION - EASTERN THAILAND -To get there take the BTS (sky train) and get down at Ekkamai Station on Sukhumvit Road.
EAST SCHEDULE AIR CONDITIONED BUS
 
MORCHIT STATION - CENTRAL, NORTHEAST & NORTHERN THAILAND - To get there take the BTS (sky train) and get down at Morchit Station (terminal station). Then you can take a motorbike taxi for about 50 baht to reach the bus station.
NORTH SCHEDULE AIR CONDITIONED BUS
NORTHEAST SCHEDULE AIR CONDITIONED BUS

 
SAI TAI MAI STATION - SOUTHERN THAILAND - To get there you can take bus number 511 from Sukhumvit Road or the Constitutional Monument.
SOUTH SCHEDULE AIR CONDITIONED BUS

 
SUVARNABHUMI AIRPORT STATION - EASTERN THAILAND &  NONG KHAI ( NORTHERN THAILAND  ) - To get there shuttle bus in the airport to the bus station area
EAST AND NORTHEAST SCHEDULE AIR CONDITIONED BUS