When entering Thailand with the aim to work, you must apply for a Thailand Work Visa, which falls under Category "B" of Thailand Non-Immigrant Visas. Despite the fact that the Category "B" Non-Immigrant Visa is given out for employment-related reasons, you cannot truly work with the visa alone. Once you are in Thailand, you must obtain a Work Permit and an extension of stay in order to be able to live and work there.
Documents required before you apply for Thailand Work Visa are listed below
Process Work Visa Thailand