Well ,there are 3 big name companies on mobile phone network service in Thailand and the name are as follow.
3 TRUE MOVE
So a tourist who come to visit Thailand and need to buy SIM for mobile phone in Thailand for making a contact with friends or family has to has one as I mention.
I don't give more detail which one is the best or better in factors of service ,price and related etc. Because mostly no significant different among them or I can say they are a stockholder who share the basic core price in services and extra services like mobile internet promotion.
But ,I would assume all you have one of them and looking for such basic information about how to access for services such as to top up the balance,to apply for internet and to check your phone number in the case you forget your own number which seem to be fun like ,but it truly happen with anyone.
Here the table collected common in use to access the mobile network services in Thailand.
Remark:Making any transactions on above are a free of charge.
English Language Problem.
When you reach up with the call center number it will be IVR system so listen carefully and press on the number ... for English. or Thai or else you have ask Thai person to assist you.
Where you can buy a prepaid card/slip for topping up your SIM balance I would recommend to get one from 7-11 store across the country and say almost at the corner of every streets.
How much for a prepaid card/slip?
You can buy initial at 50 baht ,100 baht,300 baht and 500 baht.
Note:if you want to apply for internet I would recommend to go to the assist from the clerk at the counter service across the country for better services and internet plan or packages available.
What I want to tell is that if your apple for internet service and forgot to turn off the data or to cancel the service you may lose all of you balance for the internet service ,so be careful with this issue.