You check the list and on top of the column is what entry your country can get, if your country is not on the list means you need a Visa from a Thai Consulate before you make your way into Thailand.
There is a 15 Day Visa on Arrival,
a 30 Day Visa Exempt Entry
a 14, 30 or 90 day Entry on Bilateral Agreement.
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%20on%20Arrival/VOA.pdf