Some people take to the food in Thailand immediately while others don't; Thai dishes can be pungent and spicy - a lot of garlic and chilies are used, especially phrik khii nuu, or 'mouse-shit peppers' (these are the small torpedo-shaped devils which can be pushed aside if you are timid about red-hot curries). Almost all Thai food is cooked with fresh ingredients, including vegetables, poultry, pork and some beef. Plenty of limejuice, lemon grass and fresh coriander leaf are added to give the food its characteristic tang, and fish sauce (generally made from anchovies) or shrimp paste to make it salty. Rice is eaten with most meals.
Other common seasonings include 'laos' root (khaa), black pepper, ground peanuts (more often a condiment), tamarind juice (nam makhaam), ginger (khing) and coconut milk (kati). The Thais eat a lot of what could be called Chinese food, which is generally, but not always, less spicy. In the north and northeast 'sticky', or glutinous rice, is common and is traditionally eaten with the hands.
Restaurants or food stalls outside Bangkok usually do not have menus, so it is worthwhile memorizing a standard 'repertoire' of dishes. Most provinces have their own local specialities in addition to the standards and you might try asking for 'whatever is good', allowing the proprietors to choose for you. Of course, you might get stuck with a large bill this way, but with a little practice in Thai social relations you may get some very pleasant results.
The most economical places to eat and the most dependable are noodle shops and night markets. Most towns and villages have at least one night market and several noodle shops. The night market(s) in Chiang Mai have a slight reputation for overcharging (especially for large parties), but on the other hand I have never been over-charged for food anywhere in Thailand. It helps if you speak Thai as much as possible. Curry shops are generally open for breakfast and lunch, and are also a very cheap source for nutritious food.
Thai food is served with a variety of condiments, including ground red pepper (phrik bon), ground peanuts (thua), vinegar with sliced chilies (nam som phrik), fish sauce with chilies (nam plaa phrik), a spicy red sauce called nam phrik si raachaa (from Si Racha, of course) and any number of other special sauces for particular dishes. Soy sauce (nam sii-yu) can be requested, though this is normally used as a condiment for Chinese food only.
Except for the 'rice plates' and noodle dishes, Thai meals are usually ordered family-style, which is to say that two or more people order together, sharing different dishes. Traditionally, the party orders one of each kind of dish, e.g. one chicken, one fish, one soup, etc. One dish is generally large enough for two people. One or two extras may be ordered for a large party. If you come to eat at a Thai restaurant alone and order one of these 'entrees', you had better be hungry or know enough Thai to order a small portion. This latter alternative is not really too acceptable socially; Thais generally consider eating alone in a restaurant unusual - but then as a farang you're an exception anyway.
A cheaper alternative is to order dishes 'over rice' or raat khao. Curry (kaeng) over rice is called khao kaeng; in a standard curry shop khao kaeng is only 20 to 30 Baht a plate.
Thais eat with a fork and spoon, except for noodles, which are eaten with spoon and chopsticks (ta-kiap) and sticky rice, which is rolled into balls and eaten with hands, along with the food accompanying it.