If you are living or working in Thailand you need to report to Thai immigration every 90 days. If you do not report your latest address to Thai immigration they will fine you and make renewal of the visa very difficult next time. You can report by post of you can report in person.
Most expats in Thailand tend to report their address in person or allowing someone else to report on their behalf. You need to complete a form for the reporting of your 90 day stay in Thailand and that form has been listed below. This needs to be completed and you passport with the last reporting page needs to be taken in as proof of having report before. You can report 1 week before the due day or 1 week after the due date. They tend to be a bit flexible if it is your first time reporting as mistakes occur.
Note that there is also an option of mailing it to Thai immigration. You need to send with it a self addressed stamped envelope with a 20 Baht stamp. You need to have copies of the passport, visa, entry stamps, departure card and you last reporting page which they gave you. Using the mail does come with risk. Many simply do it themselves or send someone else to do it for them. Note : This is not for a multiple entry Thai visa but for a 1 year visa that has been extended where you needed to show an income. This can be a marriage visa, retirement visa or a work permit.
Late Reporting : 200THB per day to a maximum of 4,000THB