Visa: If you are planning to come to Thailand for a period of less than 30 days you will not need to apply for a visa and will be granted a visa on arrival. However, if you wish to stay longer than 30 days you will have to apply to the Thai embassy in your country to get a longer visa.
Rules vary from country to country, it is best to check the Thai Embassy website in your own country to be sure what the rules are for you. It is recommended you apply for a visa well before your departure date.
More information is available at the UK Thai Embassy website