Bangkok is a hub for many destinations in South-East Asia and, as a result, there are a large number of airlines flying directly to Bangkok from Europe, USA, Australia and Asia. If you plan to travel around the Christmas and New Year period book well in advance since this can be a busy time of year.
Trains run between Singapore and Bangkok, with stops along the way in Malaysia, before entering Thailand at Padang Besar. You can choose first or second class sleeper accommodation, or third class (seat only). The trains are quite comfortable and are a good way to travel if you are on a budget.
Domestic air services are operated by Thai International,, Air Asia, Bangkok Airways and Orient Thai with regular services to most major destinations. Aircraft are modern, service is good, and the fares are competitively priced.
The train service run by the State Railway is a pleasant and cheap way to travel. From Bangkok there are lines to Chiang Mai and Nong Khai in the north, Ubon Ratchatani in the north east, Kanchanaburi to the west of Bangkok, and Had Yai in the south. The trains will stop at most large towns along the way, but some of the express services have limited stops, so check before buying a ticket. Most hotels will have a train time table and will happy to book you a ticket.
Air conditioned buses, both state and privately owned, run to most major destinations, and are generally the most economical way to travel. On long overnight journeys the bus will usually stop somewhere en route and passenger are treated to a free meal. For destinations off the beaten track the only choice may be one of the slower non-airconditioned buses. Most hotels will have facilities for booking tickets.
If you are travelling within Thailand during April around the time of the Songkran festival book well in advance. Many Thais travel back to their hometowns at this time so getting a seat may not be easy.