While normally I advocate doing things on your own here, I tried setting up a company myself a couple years ago and got stuck. Find a legitimate lawyer, not one of those half baked Thai agencies.
However, if you want to try doing it yourself and you're in Bangkok, go to the Ministry of Commerce (here's the link) and ask them about setting up a business. They are very friendly and there are a few people who speak English. The 11th floor deals specifically with foreigner owned business in Thailand and will give you all the paperwork you need.
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