As a general rule, you should be ok with the standard travel vaccinations. Hepatitis A and Typhoid are an absolute must for any tropical country and you get them in a combined injection. If you get a booster within 12 months, the Hepatits A lasts 10 years while you need to get your Typhoid updated every three years.
Another vaccination that is recommended is the combined Tetanus/Diptheria which lasts ten years. In certain areas of Thailand (including the far north I think), you will also need anti-Malaria tablets.
The above should be enough if you are just visiting as a tourist for a couple of weeks but if you are planning to stay say 4-5 months, there are other vaccinations that should be considered e.g. Hepatitis B but it depends on what you are doing and where you are going etc.
A lot of people think you have to go to the Tropical Medical Bureau (TMB) for vaccinations; obviously if you live in the vicinity of one of their centres, they are the best place to go but I always go to my GP for my travel vaccinations and they know my medical history and are are very knowledgeable with regards to travel vaccinations too.