If you're travelling on a UK passport, you don't need to apply in advance. You will get a 30-day tourist visa on arrival. If you need to stay longer, check the requirements.
I am not a doctor - take professional advice before travelling if you have any special medical requirements. The usual array of vaccinations for far-Eastern travel is recommended:
Travellers' cheques and credit cards are accepted in most places; some ATMs will accept some credit cards, but you may have to hunt for one that works. There's no point in taking Thai currency with you, as all the banks will change sterling, dollars and most other Western currencies, and don't take much commission. They all offer exactly the same rates, so there's no need to waste time shopping around.
Note the colours and denominations of the notes - it's easy to confuse a pink B500 with a red B100 in a dimly-lit restaurant or bar. It's a good idea to keep a supply of the smaller B20 (green) and B50 (blue) handy in a shirt pocket, for paying taxis etc. without exposing the extent of your wealth.
Apart from the heat, the first thing to strike you will probably be the smell - not unpleasant, but a striking combination of lemon grass, spices and perhaps an occasional whiff of drains. Next, the dust and traffic fumes - Bangkok is probably the world's largest permanent traffic jam .
On the plane you should have been given a customs declaration and an immigration card and filled them in. At the immigration desks, be prepared to be patient. There are usually long queues, but they move slowly and steadily. Your passport will be stamped with a 30-day visa, and they staple the departure half of the immigration card into it, to be collected when you leave.
After collecting your baggage and going through customs, change some money at any of the banks in the arrivals area - they all offer identical rates, so there's no need to shop around, just choose the one with the shortest queues. The object of changing money here is to have some small change for paying taxi fares, tips etc: if you draw cash from an ATM, you will end up with a wallet full of 1000B notes which taxi dirvers will find hard to change. Now might be a good time to tuck a couple of notes in your passport to pay the airport departure tax (B500) when you finally depart.
If you happen to pass by the TAT (Tourism Authority of Thailand) stand, it's wortk stopping to to pick up a free map of Bangkok.
If your hotel provides a free "limousine" service, there should be someone there holding up a sign with the hotel's name. If not, the recommended way to get to your hotel is by metered taxi. Alternatives are the airport limousine service (unnecessarily expensive), unmetered taxis (fixed fare, agreed in advance) and unlicensed taxis. The latter may (rarely) be cheaper, but arenot recommended. There are horror stories of unwary passengers, new to the delights of the East, being held up and robbed.
To find the meter taxis, go downstairs from Level 2 (Arrivals) to Level 1 (ground level) and find the clearly-labelled "public taxi" desk. This is not the same as the "airport limousine" desk on level 2, which will cost you much more. Do not accept the many offers of transport you will receive from touts en route, and do not be swayed by "official" looking uniforms and photo-ID badges. All these people are looking for a cut of the commission you will end up paying.
If the queue at the public-taxi desk is very long another option is to go back up to Level 2 and take the free airport shuttle bus to the Public Transportation Centre, where there should be another public-taxi desk and more taxis. This is not the same as the fixed-fare "airport bus" routes which link the airport to some city-centre locations.
At the public taxi desk, tell the staff your destination, and they will give you a document for the driver with your destination written in Thai. Show it to the driver, but don't let him keep it, as you may need it in case of complaints. The next available driver will then take you there. Make sure that he turns his meter on! On arrival you pay the amount on the meter, plus a 50B airport surcharge, rounded up to the nearest 10 baht as a tip. The distance from Suvarnabhumi to central Bankgok is around 40 km, which should cost less than 250B on the meter unless you are held up in traffic. At certain times of day the driver may ask if you want to take the expressway: say yes. You will also have to pay the expressway tolls (aound B65) and if he has no change, the driver may ask you for money as you approach the toll points. If not, he will add the toll to the final fare.
An alternative is to go upstairs to the departures area, where you will be able to pick up a meter-taxi who has just dropped a fare. Again, be sure he turns the meter on. I'm told that he may make a small extra charge for airport pickups - apparently this is legitimate.
See also the section on transportation in general.