Taxis
Getting a cheap, legit taxi at the airport is not difficult, but it does require some preparation and your full attention if you want to avoid the ubiquitious scams that the Western farang fall prey to every day upon their arrival into Bangkok, especially since weary-eyed, exhausted, and travelers not familiar with the local culture are the ideal targets for unscrupulous locals to take advantage of to make a few extra bucks. These con-men (and women!) are well-aware of how inexpensive Thailand can be compared to most countries and know that tourists can easily fall into the trap of believing that the price they're paying is a bargain (and it usually is, relatively speaking), but unless you're in a metered taxi, you will be paying far more than necessary, and even if that's not a major issue, you should seek out a metered taxi for your own safety and peace of mind.
When you first arrive in Bangkok and exit customs, you'll be approached by men and women right and left asking if you need a taxi, and while you may feel like you're being impolite by saying "no" to them over and over, just know that these people are not looking to help you but more likely to con you. You'll be on level 2 when exiting customs, and just look for the nearest escalator or elevator and go down to the ground floor, which is where you'll start to see the signs directing you to a metered taxi. There will be a line (don't worry, it moves quickly) or at least a sign that leads to a table with a person sitting down. Tell him or her your destination, which will then be written down on a slip of paper and given to you to give to the driver of whichever taxi you'll be led to. The taxis are a garish hot pink color, and easy to spot lined up on the ground level. Once your bags are loaded in the car, verify that it's a metered taxi ride (a simple gesture like pointing to the meter will do), and you'll be on your way. Enjoy the ride into the city and marvel at the meter, which you'll at first think is either broken or not turned on because the fare is ridiculously low and remains that way, but don't worry--this is the reality of the Thai taxi--and yes, you really can drive for fifteen minutes at high speed before the fare goes above the equivalent of $1 USD.
Taxis in Bangkok are cheap, they mostly run on propane. They are usually modern cars like Toyota Caronas (or similar). The taxi drivers run on meters and you should always ask for them to turn on the meter. They charge only in Baht (local currency) and the taxi rides are approximately $3.00 for every half hour driven. You can get lost in a taxi for well over 1 hour and the total taxi fare was $6.00. You will find them easy to use and a better way of transport than a tuk tuk if you are traveling distances that are over 10 minutes. Unlike tuk tuks, you will have a meter running instead of negotiating a rate.
Be aware that you will be responsible for paying any tolls in addition to the meter fare, and it's between you and the driver to decide if you'll just pay at the end or if the driver wants you to front the Baht each time you stop at a toll booth. Going to and from the airport you'll stop at around 2-3 toll booths, each costing around 30-60 Baht, and the total amount you'll spend on tolls will be approximately 100-150 Baht. The amount will be printed on a little ticket that the driver collects at each stop, so if you plan on paying for the tolls all at once, hang onto these tickets and then tally up the amount you owe once you arrive at your destination. Try to be prepared and helpful by having some small bills or change with you to cover the tolls, which you should keep in mind if you're exchanging currency at the airport. By default they'll give you large bills only, so be sure to ask for some smaller denominations for this reason. It's also useful to have small bills for tipping, and the drivers expect to be tipped at the end of the journey. Sometimes this can be a matter of rounding up the fare and letting the driver keep the change--a tip of 20-40 Baht will be appreciated, and even though it may seem like very little compared to Western standards, a Thai local can live off of 20-30 Baht per day.
As an aside, you can charter a taxi all day to see the sights of Bangkok and have your own private driver for approximately 1,500 Baht. That comes out to approximately $37.50 for nine hours. Have fun and enjoy the wonderful inexpensive private taxis of Bangkok.
Lastly, don't be penny-wise and pound-foolish when it comes to taxis; they're very cheap and with the sweltering heat and humidity, you'll never appreciate air-conditioning more than when you're getting around Bangkok.
Rental Cars
Unless you can read Thai script, know your way around the city, don't mind driving on the left side of the road, and like paying lots of money for no practical reason, you shouldn't think about renting a car in Bangkok and just stick to the comforts of someone driving you around in a taxi, which is so irresistably inexpensive that it would make no sense to rent a car.
