When do I need an inoculation before going to Thailand?

You don't need any to visit Thailand. Hep A and typhoid are useful for even your home country so you could get those if you like and you have time. Also up to date tetanus.

The passport you enter Thailand with determines if you can enter free for 30 days with a Visa Exemption stamp at Suvarnabhumi or not. Other countries require a visa you pay for so know what you have to do ahead of time.

Some people get shots and some do not. I have free medical care so I get quite a few. Do what your doctor recommends. Hepatitis A & B, Tetanus and Typhoid I always keep current. When I am near the border areas I use Doxycycline for malaria. Doxy is cheap and you can buy it in Thailand. But some of these shots require you to start ahead of time.

When you ask your question on these travel forums you will get a little of everything from don't do it, they never do it or get everything. I would rather get my shots and not take a chance.

Virtual Tourist

http://forum.virtualtourist.com/forum-1943-1-Trave...

Trip Advisor

http://www.tripadvisor.com/ShowForum-g293915-i3686...

Lonely Planet Thorn Tree Forum

http://www.lonelyplanet.com/thorntree/forum.jspa?f...

Thai Visa Forum

http://www.thaivisa.com/forum/index.php?act=idx

Do what your doctor recommends.