Eating local food from street vendors is one of the most authentic experiences you can have in Thailand. It is here where you find genuine Thai food, not in 5-star restaurants. If you step outside your comfort zone, you can find amazing choices of food. 

Thai food is heavily influenced by Chinese, Malay, Indian, Lao, and Burmese cuisines, but there is also a western influence that even Thais are mostly unaware of. During the Age of Exploration, when Europeans "discovered" (at least from the European point of view) the Americas, Asia, Africa, and Australia a widespread transfer of goods, culture, technology, and ideas happened. At that time chili, tomatoes, corn, papaya, pineapple, peanuts amongst other foodstuffs were introduced to Thailand from South America.

Thai cuisine varies from region to region with different local specialties. In general, the northeastern food (Isaan food) is known to be very spicy, the central plains (Bangkok) are home to soups and use a lot of coconut milk, while the southern food tends to be influenced by Indian and Malay food.
 
Ordering food in Thailand can become a little overwhelming for tourists.  It is relatively easy as long as you're in Bangkok, Chiang Mai, or Phuket and eat where the staff speaks English. But once you explore more remote locations or order your food at a food stall, a bit of Thai comes in handy. In the following article you'll find a simple way to order a variety of Thai dishes.

First, let's look at how you can politely request something in a store or restaurant. This also includes a small excursion on "Polite Particles", a central element of the Thai language. You can read more about polite particles here.

On a side note, the transcription from Thai into English is arbitrary -- there simply is no standardized transcription, and if you compare transcriptions in Thai language books and dictionaries you will find some variety. 

Back to our polite request. If you want something, e.g. food or goods in a shop, this is how you say it in Thai:

เอา   +  [something]  +  ค่ะ /ครับ

Here are four typical Thai meals:

If you would order one of these preparations, the conversation would most likely go as follows:
You: I'd like fried rice, please.
เอา ข้าว ผัด ค่ะ .

Waiter: Which fried rice do you want?
เอา ข้าว ผัด อะไร คะ ?

Waiter: We have chicken fried rice, shrimp fried rice, and vegetable fried rice
มี ข้าว ผัด ไก่ , ข้าว ผัด กุ้ง , ข้าว ผัด ผัก ค่ะ . 

The easiest way to order is therefore to combine the preparation with the main ingredient:

Of course, rice is the main ingredient added to fried rice. But Thais usually add another ingredient like meat or seafood. If you want no meat, it is a good way to say "vegetables" instead. Here is some ingredients that you can use:

Now we can expand our polite request to include the preparation and the main ingredient:

เอา   +  [preparation]  +  [ingredient]  +  ค่ะ /ครับ

This is the perfect way to order food in Thai.

Let's look at some examples:

Not spicy, please:
ไม่ เผ็ด ค่ะ /ครับ
ไม่ means "not". เผ็ด means "spicy"

Don't add MSG, please
ไม่ ใส่ ผงชูรส ค่ะ /ครับ
MSG is ผงชูรส . ใส่ means to add or put something.