Your High-School Diploma is not from a Thai School?
In this case, you need to get a confirmation in your country that your high-school diploma is equivalent to the U.S. grade 12 high-school diploma, or the British university entrance diploma, or Mathayom 6 of Thailand. (In case you want to study non-degree, please certify that your high-school diploma is equivalent to the U.S. grade 9 Junior High-School Diploma).
Certifying authority can be any accredited educational organization, such as state or state accredited schools, universities, or the ministry of education in your country.
A signature and an official seal is needed for credibility. Please be prepared to submit the original document with your application papers.
If necessary, you can get this certification also in Bangkok, either at the education section of your embassy or in the Ministry of Education. Besides the effort that needs, especially requesting the certification at the Ministry of Education is a time consuming process. Since they will send a letter to your school and then wait for the answer, you should consider about two months until you have the paper in hands. You certainly can already start your studies, but the uncertainty remains over this period of time.