@Kim *******
As a foreigner, you need three things to legally work as a teacher in Thailand.
1. You need a visa that allows work (non B or non O based on kids/marriage as examples).
2. A work permit.
3. A teaching license.
You need as a minimum to hold a bachelor's degree, in any field.
You can transfer your existing license from California.
You would need notarized and legalized copies (and originals) of all documents, including diploma and transcript of your highest education and your teaching license if applicable.