Having grown up in the States, I was definitely accustomed to the American food culture and a specific way of eating. After moving to Thailand a few years back and getting in touch with my roots, I had no choice but to adapt and learn some new dining habits. If you're planning a trip to Thailand in the future and don't want to look like a total amateur, this is guaranteed to help you eat like a Thai person.
Fork and Knife VS Spoon and Fork
If you're from any Western country then you're probably used to eating with a fork and a knife, but here in Thailand, you use something else: a spoon. Yes, even for dishes without soup. Instead of jabbing at your food with your fork, this utensil will have more of a supporting role. Use it to push food onto your spoon and eat with that instead.
Eating Whenever VS Waiting for the Eldest Person to Begin
Like most Asian cultures, people in Thailand have an immense amount of respect for their elders. Before you start eating, you should wait for the eldest person at the table (even if they're only a few years older than you) to begin before eating yourself.
Splitting the Bill VS Paying for Yourself
I prefer going dutch personally, but when in Thailand, do as the Thais do. In the States, it'd be no big deal to split a bill down the middle, but here, you're expected to pay for you and you alone, even if it's only a few baht (unless you're the host, in which case, you're expected to pay for it all.)
Eating Alone VS Eating In a Group
This might sound strange, but in Thailand, eating alone is actually considered bad luck. You'll often see people going out to eat in groups and this is actually one of the best ways to meet people in Thailand. Ask anyone if they'd like to grab a bite to eat and odds are they'll say yes.
Ordering a Main Course VS Sharing Different Dishes
Similar to Spanish tapas-style dining, in Thai dining, people will typically order a variety of different dishes to share with one another. Typically there'll be a meat dish accompanied by a few vegetable or curry dishes (served with rice, of course). Each dish will come with its own spoon so for the sake of those dining with you, make sure to use that to serve yourself and keep the utensils you're eating away from others.
Huge Portions VS Smaller Sizes
Since most people order a variety of dishes to share, it's quite common for authentic Thai food to come in smaller portions than their American counterparts. Even if you aren't dining in a shared-plates environment, you'll still find relatively small portion sizes, but it's totally normal for people to order another round if they're still hungry!