If I declare my income, would I have to pay tax on it? I work and earn this money outside of Thailand. How do I go about applying for the marriage visa?
If you will used and declared your monthly income for this marriage visa application, you will not be obliged to pay taxes as you have earned this income from outside Thailand.
The only thing that is required from you before coming to Thailand is evidence of your income to obtain the letter from the embassy. An income affidavit from your embassy in Bangkok should be obtained first in order to proceed together with other documents proving you married to a Thai national.
A non immigrant “O” visa should be applied for as well from the Thai embassy abroad which will be extended to 1 year inside of Thailand.