Sometimes. Citizens of the UK, Australia, New Zealand, most EU countries, Canada and the United States do not require a visa to visit Thailand for up to 30 days. If entering Thailand by land border then they will usually only be granted a visa valid for 15 days.
EU citizens and other nationals not listed above should check with the Thai Embassy in their country of residence for the most up-to-date information.
In the event of a visa being required (see above), arrangements should be made with the Thai Embassy in the advance of your departure to Thailand. It is usually a single entry visa that allows you to travel in the country for 60 days. Many embassies offer an express service as well as a multiple entry visa, both at additional cost.
You will need one clear page (per visa) in a full passport that is valid for at least 6 months beyond the date of your departure from Thailand.
Proof on onward travel is required if flying into Thailand on a one-way flight (i.e. without an exit flight from Thailand). If proof is requested, your Selective Asia itinerary is sufficient proof of your onward travel arrangements.