If you want to work in Thailand, the normal procedure would be to apply for a B visa in your country of residence, with a Letter of Approval from the Thai Labour Department. After arriving in Thailand, you would then apply for your work permit. The work permit can also be requested by your employer before you arrive.
If you enter Thailand on another visa (i.e. a tourist visa) and then look for a job, you are unlikely to receive a work permit. However, you have more of a chance if you are offered a job by a company which is legally allowed to employ foreigners (yet this is still not guaranteed). In either case, you will still have to leave the country to apply for a B visa before you can apply for your work permit.
To apply for a work (B) visa, you will probably need some or all of the following:
- A fully completed visa application form
- Passport or travel document valid for at least 18 months
- One (4 x 6 cm) passport photo, taken within the past six months
- Evidence of adequate finance
- Letter of Approval from the Ministry of Labour (to obtain this letter, the prospective employer in Thailand is required to submit Form WP3 at the Office of Foreign Workers Administration)
- Letter from a company stating the objective of your visit to Thailand
Spouses and children of a B visa holder will need a non-immigrant O visa, which is also issued to foreigners who wish to retire in Thailand.
For more information check the website of the Ministry of Foreign Affairs.