It is difficult to give you WHAT to order and HOW much because
1 - I don't know where you are (what country, where in that country)
2 - I don't' know what you like to eat (are you a meat eater or mostly vegetarian, can you eat spicy food, etc)
3 - Not every restaurant will have the same dishes on their menus
4 - different restaurants sure will have different prices and the higher class restaurant will be more expensive of course.
Tell them that you never had Thia food before and ask the waiter for suggestions. Tell them if you can eat hot spicy food or not and if you can't make sure the waiter know NOT to order anything with chilli in it.
Generally - for beginners:
- spring rolls with dipping sauce or satay chicken sticks to start.
- two stir fried dishes - one chicken one pork (ask waiter for suggestion ). One dish could be a little spicy and the other with stir fried veggies.
- a curry (masaman curry is the mildest, or try red curry)
- some steam rice to go with the dishes.
But you can also try Pad thai - as many people have suggested (but if you are going to have rice and curry - then don't order pad thai - as it is noodles and no point ordering a noodle dish if you are going to have rice)
But...for a better experience:
Be adventurous! Try something exotic...Thai food is aromatic and flavoursome. Your tastebuds will love you forever.
And note: Thai food is eaten with spoon and fork and NOT knife and fork. If they give you knife and fork on the table - they are NOT Thai.
Cheers from Australia