Thailand entry rules depend on passport and can change. Many nationalities qualify for visa-exempt entry. For example, Thailand has expanded visa exemption for many passports to 60 days with an option to extend (check current eligibility and extension rules with a Thai embassy or consulate).
Thailand also uses an online arrival card system (TDAC). Plan to submit the required arrival details in advance (the official guidance commonly references submission within the last 3 days before arrival). Keep the confirmation available for immigration.
General best practices: travel with a passport that has at least six months validity remaining, carry proof of onward travel, and keep your first-night address handy.