If you live in Prakhanong then you will need to contact the Amphur Prakhanong to register your marriage in Thailand. You will require an affirmation agreement or freedom to marry letter which you will need to get from your Embassy in Thailand. If you are going to register a prenuptial agreement then you would have to speak to a lawyer in Thailand for more information as they have to be done at the same time.
There are a number of district offices in Thailand and you will need to use the one which is where you live. They will ask you about this and you will need to take proper advice on this. There are a number of amphur offices in Thailand then you will need to check where they are and which district they cover. Note that your documents need to be certified and translated by the department of Foreign Affairs before you go to the amphurs office.
Khet Prakhanong Registration Office
1792 Sukhumvit Soi 54, Kwang Bangjak, Bangkok 10250