The two main opportunities for work for foreigners are teaching English and dive instructor, but both are very competitive and dive masters in particular are paid a pittance.
To become a dive instructor, the most popular destination is Ko Tao (Turtle Island) a few hours off the coast of Chumphon in the Gulf of Thailand. There are dozens of dive shops that provide training and internships.
Anyone with a four-year degree can gain employment as an English teacher in Thailand, and even those without a degree can usually find work under the table. Normal starting salary is approximately 30,000 baht per month and this goes up and down slightly depending upon location (higher in Bangkok, lower in some up-country towns).
One way to start working as a teacher is to gain a TESOL/TEFL Certificate. One of the largest TESOL schools in the world is head quartered in the small village of Ban Phe, Rayong. Other provinces in Thailand offer TEFL/TESOL Certification Courses. In Northern Thailand, Chiang Mai University  has a comprehensive teacher training program located on its main campus.
Finding any other kind of work in Thailand can be difficult, as wages are poor and a large number of occupations are legally off limits to non-Thais. Thai law requires that a foreigner earn quite a high wage to be eligible for a work permit. Companies and schools may assist employees to obtain visas and work permits, but some schools avoid the extra work involved.
Volunteering is a great way to meet locals and experience the culture and traditions of Thailand. There are many worldwide organizations that offer volunteer work on such projects as community development, conservation, wildlife sanctuary maintenance & development, scientific research, & education programs. And they won't ask you for money! Here are three of them:
Important: Volunteering is defined as a form of employment by the Thai authorities. Foreigners must obtain a work permit even to volunteer for small projects. This is easier to obtain than a normal work permit, and can be issued even for one or two days. Tourists are advised to take these rules seriously. Thai jails are not comfortable; if you are arrested on a Friday you may not be able to contact anyone before Monday.