Foreign nationals need a valid work visa to work in Thailand. The Thai Ministry of Labour only issues work visas to foreign nationals who have a qualification that is in high demand in Thailand. The prospective employer must also provide proof to the immigration authorities that no Thai nationals are available for the job. Finding a well-paid job in Thailand is difficult.
It can take several months for a work visa application to be approved or denied. You are not allowed to work while your application is being processed. Violations can result in severe penalties. Foreign nationals who have a job offer and have entered Thailand with a 'non-immigrant B visa' issued by the Thai embassy or consulate can ask their prospective employer to apply for a work visa on their behalf.
Once you are in Thailand, you will need to obtain a work visa from the Royal Thai Immigration Bureau and have it affixed to your passport. The Immigration Bureau is also responsible for visa extensions and renewals. Immigration offices can be found in all larger towns and cities.