Technically , there is no such thing as a work visa.  The proper name is "Non-Immigrant B Visa".  
Once this Visa is obtained, you then apply for a Work Permit


STEP 1 : OBTAINING A NON-IMMIGRANT VISA


REQUIREMENTS FOR GETTING A NON-IMMIGRANT VISA OUTSIDE THE COUNTRY:
.
The applicant has been offered a job or starts a Thai company to employ himself/herself
The company requests that the applicant be given a non-immigrant visa so the company may apply for a
Thai work permit for him/her
The company knows the person to be dependable, upstanding, and law abiding and that they will respect
the laws and customs of the Kingdom of Thailand.
You should apply for this visa within 30 days before you plan to depart your country. The Consulate or
consular officer will ask for copies of the registration documents and financial statements from the
company.


STEP 2 : OBTAINING THE THAI WORK PERMIT.
    Download Work Permit Form

Apply for the work permit at the Labor Department covering the province in which the Company’s Head
Office is located. (Employees working in a Branch office in other provinces must still apply in the province
where the Head offices is located).
Most people get refused because they forgot to include a map in Thai showing the location of the
company.


EMPLOYEE PROVIDED DOCUMENTS:

Passport - copies of every page. Each copy must be signed by employee.
Non-Immigrant Visa
Departure Card TM.6
Education degree (signed copy) *
Transcript (signed copy) *
Certificates or licenses held by applicant (signed copy) *
CV or Resume – describe in detail the applicant’s past positions, duties, performance, length and place
of employment.
Photos, three (3) in quantities which are 5 x 6 centimetres in size (not passport photos) with full face and
taken wearing business attire (no hat and some jurisdictions require suit and tie). This must have been
taken within six (6) months before the application for the Thai work permit.
Marriage Certificate (if married to Thai National). This includes the original and signed photocopies. Also
include wife’s Thai ID card, birth certificates of children, and household registration.
.
* Thai government officials may require these documents to be certified by your country’s embassy (this
requires bringing your degree, resume, license or certificate to your embassy to declare it is a true and
original document. You are required to pay an authentication fee to your embassy. In addition, they can
require you to translate these documents into the Thai language from your foreign language.

...
EMPLOYER PROVIDED DOCUMENTS:
...
Commercial Registration Department Certificate showing that the organization for which the applicant is
going to work has been duly registered as a juristic person, giving the name of the Managing Director
and/or Director, and its objections and registered capital.*

Shareholders List certified by the Commercial Registration Department.*
...
Factory License (if required) issued by Factory Department, Ministry of Industry.*
...
VAT Certificate - Phor Phor 20*
...
Withholding Tax – Phor Ngor Dor 1 (if renewing work permit)*
...
* Thai government officials require that all documents to have the seal of the company stamped on every
page and the true and authorized signature(s) of the Managing Director and/or Directors next to the
seal. In addition, government officials have in past requested official copies of registration, shareholder,
licenses and certificates to have been issued by their respective agencies within the past 90 days of your
application for Thai work permit.


STEP 3 : THE TAX ID CARD:


THAILAND REVENUE DEPARTMENT:

Once your Thai work permit has been arranged, the next step is for your company to apply for a Thai
taxpayer identification card. This ID card has a tax ID number which you will need to list on tax documents.


STEP 5 : RENEWING YOUR VISA AND WORK PERMIT:


YOUR WORK PERMIT EXPIRES WHEN YOUR EXTENSION OF STAY DOES

You will need to renew your stay in Thailand before you can renew your Thai work permit. This can be
done in Thailand or in some cases you must leave the country. If you cannot obtain your extension inside
Thailand, you will have to get a new non-immigrant visa from a Thai Embassy or Consulate abroad as
explained above. If you have an unexpired multiple-entry visa, all you have to do is go over the border
and then return. If required to go outside Thailand to obtain a new visa, you will need to provide the
Embassy or Consulate with copies of your work permit and new recommendation letter from your
company, company registration documents, and financial and tax reports. Siam International Legal
Group with its global presence can assist you in any country with renewing your visa. Again, all of this
must be done prior to your work permit expiration.



REPORTING YOUR STAY EVERY 90 DAYS

For persons on a long term extension of stay, they are required to report to Immigration Police every 90
days. Persons holding a multiple entry visa will simply depart Thailand and renew their extension of stay
after each 90 day entry. Our firm offers professional service with a yearly package to perform the task of
the reporting to Immigration each 90 days.


You need to apply for a non-immigrant B visa, which allows you to apply for a teacher’s licence, etc.
Many people take the option of returning to their home country because non-immigrant B visas can be
more difficult to obtain in the Asian countries bordering Thailand, especially if you don’t have the correct
paperwork.

References

Ajar.com
  http://www.ajarn.com/help-and-guides/work-permit-visa-faq/

Here's an excellent article from  Godfree at Thailand Retirement Helpers
  "><godfree@thailandretirementhelpers.com>
Don's Life In Thailand (Chiang Mai)
Work Visa
(Non-Immigrant B Visa)