Well, pad thai is very popular, but in my opinion it can be quite terrible if poorly made so I would never order it somewhere I didn’t know. But it is a crowd-pleaser so perhaps you would love it. It’s a noodle dish, the noodles are stir fried with onions and spices, egg (sounds odd but it works), and meat (usually chicken and/or shrimp) and/or mixed veggies. Sometimes tofu is in there too. Sometimes there is fresh cilantro on top. Often there is ketchup used in the cooking, but pad thai is best when the tomato sauces are skipped in my opinion. There may be a curry pad thai option on the menu, in which case I encourage you to order that instead of the plain pad thai: it’s usually better.
There will also likely be a variety of curries made with coconut milk and curry paste. Order these with rice if they don’t come with some because they are usually quite brothy. Usually you can get shrimp ones, beef ones, chicken ones, or vegetarian ones. They are made with either red, green, or yellow curry paste. If you are new to Thai cooking I think you should avoid the green curries: try a red or a yellow instead. The green is quite deep tasting; it takes some getting used to.
Spring rolls are a great appetizer, of course. Thai spring rolls are usually smaller and more compact than Chinese-style spring rolls. There may also be salad rolls (maybe called nam dip, or fresh rolls, or any number of other things). These are amazing when they’re amazing, but too often they’re stuffed with dry noodles or iceberg and are not good so be wary.
Sticky rice with peanut sauce is another wonderful appetizer, although filling. Share it with someone or have it and a salad as your entree. (Unless you usually eat a lot of food.)
If you order rice with your meal, you’ll probably have the choice between plain rice and coconut rice. Go for the coconut rice! It is rich and sweet. (But not too rich or sweet.)
If there is a dish called ping gai order it! It is chicken, marinated in a variety of spices (garlic, ginger, coriander, red chillies, amongst others), then BBQd (or grilled or thrown under the salamander, whatever). It may not be called by name so look for it by description. It may be served with a side salad and side rice and a dipping sauce. It is so yummy—I said pad thai was a crowd-pleaser before, but this one really is a crowd-pleaser! I always tell people “if you like chicken, you will love it,” and it’s always been true. (Although sometimes people remove the skin, they still love the rest of the dish.)
There will also probably be a dish called basil chicken or basil beef. These are stir fries that use “thai basil” (although they may just use normal basil). Thai basil is stronger than normal basil, but still yummy. (You might not want to eat the actual basil, but it flavours the meat wonderfully). This is a simple inoffensive dish that is good to order if the menu overwhelms you. Order it with rice if it doesn’t come with any.
There may be some amazing soups. Tom som gai is a chicken-broth soup with lemongrass, chicken, and perhaps some mixed vegetables, maybe with a little coconut milk thrown in. It is usually very spicy but really, really good. There may be some coconut milk soups as well, which are less spicy but very, very rich. There may be hot and sour soups, these are different from the Chinese-style ones, likely more glutteny. I don’t like them much myself.