If you are over the age of 50 then you can apply to live in Thailand for one year at a time using a Thai retirement visa. The application process starts with obtaining a 3 month Thai retirement visa also known as a “O-A” visa. This is extended in Thailand.
If you are married to a Thai then you can apply for a Thai marriage visa and have this visa extended in Thailand for a period of 1 year at a time. The visa is much better than having a tourist visa as you can apply for a work permit while having this visa.
Depending on what you wish to do in Thailand you would need to supply the Thai Embassy with the documents needed for what it is that you wish to do. Teaching, owning a business or investment. These are the basics for the Thailand business visa.
If you are visiting Thailand then you might be in for a surprise as many of the immigration rules have changed. If you arrive in Thailand by air then you will receive 30 days visa on arrival, however arriving via land that has changed.