I was told that there were two requirements; 1) I had to be married, 2) I had to have been resident over 12 months. To apply, translated and certified copies of passport and original 'full form' birth certificate. My U.K. birth certificate was a 'short form' which didn't state parents. May I suggest you visit your Amphur/City Hall and ask them for a list of their particular requirements. As already advised above, it is unlikely they will speak English well enough to explain so taking a Thai person with you will help.