Visiting Thailand it is always best to apply and obtain a tourist visa before you visit Thailand. Many still today arrive in Thailand with a visa on arrival however since the visa changes 3 years ago most now arrive with a tourist visa. Arriving via a border crossing in Thailand without a visa gives you 15 days stay in Thailand. If you however are arriving via an international airport you now get 30 days.
The tourist visa gives you 60 days in Thailand no matter which way you arrive in Thailand. This 60 days can be extended in Thailand at Thai immigration for another 30 days for the fee of 1,900THB. You can extend it one more time again for 1,900THB however you will only get 7 days. That is the last extension that can be done on the tourist visa.
The tourist visa can only be used for tourism and cannot be easily converted for anything else. If you are married to a Thai the marriage visa is a much better option. If you are doing business in Thailand such as attending business meetings then the business visa is much more suited.
If you want to know what your options are in Thailand with regards to visas and staying or visiting Thailand then speak to an immigration lawyer in Thailand for more information.