Public Bus
If you don’t mind a little hassle and have time to spare, public buses are the cheapest way to get downtown. Outside the Arrival Hall, at Entry Gate 5, locate the Airport Shuttle Service stand and hop on the express shuttle to get to the Public Transport Centre located outside the main terminal building. Choose your bus from one of the 11 available routes. The rates are 24–35 baht, depending on the distance. Each bus makes from six to eight stops along the way, so the journey will take more than one hour. Please note that only some routes operate 24 hours.

Public Van
Compared to public buses, these vans make fewer stops and offer a little more comfortable ride. You will need to take the express airport shuttle service to the Public Transport Centre. Choose from the nine available routes. Rates range from 25-70 baht. Please note that most routes do not operate 24 hours.

Car Rental
You will find Avis, Hertz, Budget, National and Master car rental counters at the International Arrival Hall (between Entry Gates 7 and 8). Service is available 24 hours.

Sky Train
There are two lines: The Silom line runs west to south, between the National Stadium in the Siam shopping area to Wongwian Yai in Thonburi (across the river), while the Sukhumvit line runs north to east from Mo Chit to On Nut. The two lines meet at Siam Station, and also interconnect at two points with the underground (MRT) – at Sala Daeng and Asok stations. A new train arrives every 3 - 6 minutes or so between 06:30 and midnight. The last train leaves between 23:30 and 23:50. Fares start at 15 baht for one stop (more info about the BTS Passes).
Trains can get pretty full during peak hours (07:00 - 09:00 and 16:00 - 19:00), as the BTS has also become the choice mode of transport for people living and working in Bangkok. 

Note that Royal Orhid Sheraton Bangkok nearest BTS Station is Sathorn Tahsin BTS Station which is on the Silom Line.

BTS Route map 
Click on it for more information