How to enter an MRT station
MRT stations are usually at ground level and then descend underground. At each entrance there's a security scanner and a guard. You have to let the security guard look inside your bag no matter how small it is.
This can be awkward and difficult with a lot of luggage but generally the guards are very polite. Simply unzipping one section and letting them shine a torch in is enough.
How to buy a MRT ticket
Firstly it's not a ticket. You will get a small black plastic circular token the size of a coin. This token uses RFID contactless technology to let you enter the platforms. You can also purchase a 1, 3, and 30 day pass directly from a person at one of the kiosks.
1) The most common method and easiest for tourists is to simply insert coins and/or notes into the ticket/token machines located just before the barriers to get a token.
Each machine has a large touch screen that is available in Thai or English. Simply touch the screen to activate it and press "English" on the top right hand corner. The screen will display a map showing the MRT route and stations. Simply touch the circle beside the name until it goes green to display the price you need to pay. The screens need to be long pressed rather than short pressed.
When you know the value of the token insert either a note into the flat slot under the coin slot or use coins for the coin slot at the top. If using notes stick with 20 baht notes that are in good condition. The machines will except 100 baht notes but you will get all your change in coin!
Once you've inserted the correct change the small black token will be dispensed into a tray at the bottom of the machine on the left hand side. After a 3 second delay your change will drop down into the right hand side of the tray.
2) You may skip the machine altogether and purchase a token directly from a person at one of the kiosks in the MRT station. This works well if the MRT is busy.
3) 1,3 and 30 MRT passes are available at the kiosks but are not great value for tourists unless you plan to take many MRT trips in one day.