1) The most common method and easiest for tourists is to simply insert coins into the ticket machines located just before the barriers.
Beside each machine is a map showing your present location and the rest of the BTS stations. Each station is represented by a name and number. On the ticket machine press the number of the station. A price that usually corresponds to the station number will appear. Insert your coins (coins only) into the machine until it counts down to zero. A thin credit card sized ticket will then appear from a lower slot.
If you only have notes go to the kiosks located beside the barriers and they will give you changed (this is normal).
2) Newer ticket machines now accept notes.
3) In the event of a ticket machine breakdown the kiosks will give you a ticket but you can't normally buy directly from them.
4) Purchase a Rabbit card or a monthly / Weekly pass for the BTS. This is possible from the kiosks but it often does not work out cheaper for short stay tourists.