Which visa will I need in order to become legal?

A Non-immigrant B Visa is generally the Visa you need to obtain if you are looking to work here and to get one you are required to produce a whole stack of paperwork including a letter of employment, an employment contract, maybe a letter from your embassy (and there is even talk of a security background check). You really need to call the embassy or consulate ahead of time and find out EXACTLY what they require if you want a non-immigrant B. If you turn up with 100 pieces of paper, then you can bet your boots the officer will ask for 101.

On entry you’ll receive 90 days of cover, with the option to extend by 10 days at Imm (I GET 14 + 6 ALL FOR 1900) (cost: 1,900 Baht). You can also obtain (although this may have changed recently) double entry Non-imm Bs (two entries of 90 days, the second entry is obtained by exiting and re-entering the country). Multiple entry Non-imm Bs are the best as they will give you just under 15 months of cover in 90 day intervals. For a one year period ANY and EVERY time you enter Thailand with the Non-imm B you’ll receive 90 days on entry. Generally most school prefer you to be on a Non-imm B if you wish to obtain a work permit, although as mentioned before a Non-imm O (spousal or dependants Visa) is in some ways interchangeable and vice versa! A word of advice most Consulates are far friendlier when it comes to issuing Non-imm Bs (and multiples at that) than Embassies. Cost of Non-imm B in the UK: http://www.thaiconsul-uk.com

Again, the immigration rules overhaul of October 2006 meant that non-immigrant visas suddenly become a lot harder to get. Well, some people have had problems….but there are people who have problems doing anything.

A Non-immigrant O Visa is generally for people married to a Thai national or with Thai children or dependants. This works in basically the same way as a Non-imm B. Although if you can show funds in a Thai bank account (400,000 baht) or show sufficient proof of local or overseas income to prove that you can support your family, the immigration can indefinitely extend your Visa for up to 12 months. A work permit can now be obtained on a Non-imm O (it couldn’t before 2006)

Generally (or it seems a lot of employers prefer) a Non-immigrant B Visa. Although it’s possible to obtain a work permit with a Non-immigrant O Visa, it seems a lot of employers either aren’t aware of this, or it’s too much hassle for them. Generally you’ll need at least a couple of months of your Visa left (which would initially be for 90 days) for the school to obtain all the needed paperwork…although if they can show they’re going through the motions Immigration can and will grant an extension (generally of around a month) to give your employer time enough to finish everything off.

Important
Many people will disagree with me on this but I sincerely believe that no visa requirement rule is set in stone. There are many ‘human’ factors involved when you apply for a visa and you are face-to-face with an immigration officer.
Is the immigration officer in a good mood?
Does he / she like you as a person and are you polite?
Are you dressed smartly enough? (you don’t need to be in a velvet smoking jacket but Thais like clean people)
Do you look like someone Thailand would want living in its country?
etc, etc - the list is endless. Most of it is just plain common sense.