How can I extend my 30-day tourist visa to 90 days in Thailand?

Go to immigration office to get extended Tod is right 1900 plus 200 for photos

If you needed longer than the 30 day extension you have two choices;

Go to a border, stamp out of thailand, stamp in&out of another country and stamp right back into thailand to get another 30 day visa exempt entry.

OR

Go to a thai consulate in a nearby country and buy a single entry tourist visa which would get you stamped in for 60 days when you enter the country (and you could extend for 30 more days as well)

there is no such thing as a one month tourist visa :/

If you arrrived here and were stamped in for 30 days you came in on a visa exempt entry.

There is no 90 day extension for that but you can get a 30 day extension for 1900baht at the immigration office that serves the area where you're staying in thailand