Thailand is known for the politeness of its people - as illustrated by its moniker "Land of Smiles".  The easiest way to be polite is to use polite particles, most commonly  ค่ะ polite word for women
See in Dictionary and ครับ polite word for men
See in Dictionary. There are other particles, but ค่ะ polite word for women
See in Dictionary and ครับ polite word for men
See in Dictionary are the most important ones. Let me again emphasise this point: Please use polite particles. If you don't use them you might sound slightly rude.
Depending on your sex, you always use a certain particle at the end of the sentence. Just to be clear, as this is different from many other languages: the ending you use does not depend on whom you are talking to, but which sex you are. 

Women say: สวัสดี Hi, Hello, Goodbye
See in Dictionaryค่ะ polite word for women
See in Dictionary
Men say: สวัสดี Hi, Hello, Goodbye
See in Dictionaryครับ polite word for men
See in Dictionary

Thais use this as a general greeting in every situation. They don't say "Good Morning", "Good Evening", etc.  Just use สวัสดี Hi, Hello, Goodbye
See in Dictionaryค่ะ polite word for women
See in Dictionary/ครับ polite word for men
See in Dictionary  for all of these. If you want more information on Thai greetings, we have written an entire article about this topic.

Women say: ขอบคุณ Thank you
See in Dictionaryค่ะ polite word for women
See in Dictionary
Men say: ขอบคุณ Thank you
See in Dictionaryครับ polite word for men
See in Dictionary

You can read about other ways to say thank you in this article.

Women say: เอา take
See in Dictionaryอัน general quantifier for small objects
See in Dictionaryนี้ this
See in Dictionaryค่ะ polite word for women
See in Dictionary
Men say: เอา take
See in Dictionaryอัน general quantifier for small objects
See in Dictionaryนี้ this
See in Dictionaryครับ polite word for men
See in Dictionary

Literally "want piece this". You usually don't say "I" in Thai.

Women say: ไม่ no, not
See in Dictionaryเอา take
See in Dictionaryค่ะ polite word for women
See in Dictionary
Men say: ไม่ no, not
See in Dictionaryเอา take
See in Dictionaryครับ polite word for men
See in Dictionary

Literally "not want". If you want to specify what exactly, you can say ไม่ no, not
See in Dictionaryเอา take
See in Dictionaryอัน general quantifier for small objects
See in Dictionaryนี้ this
See in Dictionaryค่ะ polite word for women
See in Dictionary/ครับ polite word for men
See in Dictionary  and point to the object. But Thais usually leave "this one" อัน general quantifier for small objects
See in Dictionaryนี้ this
See in Dictionary out, if it can be understood from the context.

Women say: เท่าไหร่ how much; not quite
See in Dictionaryค่ะ polite word for women
See in Dictionary
Men say: เท่าไหร่ how much; not quite
See in Dictionaryครับ polite word for men
See in Dictionary

Just hold the item or point to the item while you ask.

Women say: น้อง you (for a younger person than the speaker)
See in Dictionaryค่ะ polite word for women
See in Dictionary (nong ka)
Men say: น้อง you (for a younger person than the speaker)
See in Dictionaryครับ polite word for men
See in Dictionary (nong krab)

น้อง you (for a younger person than the speaker)
See in Dictionary means "you" for a younger person, but is also used for people that serve you. Don't worry, using น้อง you (for a younger person than the speaker)
See in Dictionary is perfectly polite, even if the waiter is older than you and you are in a fancy restaurant.

Women say: คุณ Ms, Mrs, Mr; you
See in Dictionaryพูด speak, talk
See in Dictionaryภาษา language
See in Dictionaryอังกฤษ English,England
See in Dictionaryได้ไหม can you?
See in Dictionaryค่ะ polite word for women
See in Dictionary
Men say: คุณ Ms, Mrs, Mr; you
See in Dictionaryพูด speak, talk
See in Dictionaryภาษา language
See in Dictionaryอังกฤษ English,England
See in Dictionaryได้ไหม can you?
See in Dictionaryครับ polite word for men
See in Dictionary

อังกฤษ English,England
See in Dictionary means English, but also England. You are literally asking "You speak language England can?"

Women say: ไม่ no, not
See in Dictionaryเข้าใจ understand
See in Dictionaryค่ะ polite word for women
See in Dictionary 
Men say: ไม่ no, not
See in Dictionaryเข้าใจ understand
See in Dictionaryครับ polite word for men
See in Dictionary 

ไม่ no, not
See in Dictionary means "no" or "not". เข้าใจ understand
See in Dictionary means "to understand"

Women say: ไม่ no, not
See in Dictionaryเผ็ด spicy
See in Dictionaryค่ะ polite word for women
See in Dictionary
Men say: ไม่ no, not
See in Dictionaryเผ็ด spicy
See in Dictionaryครับ polite word for men
See in Dictionary

ไม่ no, not
See in Dictionary means "no" or "not". เผ็ด spicy
See in Dictionary means "spicy"

Women say: ไม่ no, not
See in Dictionaryใส่ put something in; put on (e.g. clothes)
See in Dictionaryผงชูรส MSG, monosodium glutamate
See in Dictionaryค่ะ polite word for women
See in Dictionary
Men say: ไม่ no, not
See in Dictionaryใส่ put something in; put on (e.g. clothes)
See in Dictionaryผงชูรส MSG, monosodium glutamate
See in Dictionaryครับ polite word for men
See in Dictionary

MSG is ผงชูรส MSG, monosodium glutamate
See in Dictionary. ใส่ put something in; put on (e.g. clothes)
See in Dictionary means to add or put something. ไม่ no, not
See in Dictionary means "no" or "not".

Women say: ไม่ no, not
See in Dictionaryใส่ put something in; put on (e.g. clothes)
See in Dictionaryถุง bag (e.g. a plastic bag)
See in Dictionaryค่ะ polite word for women
See in Dictionary
Men say: ไม่ no, not
See in Dictionaryใส่ put something in; put on (e.g. clothes)
See in Dictionaryถุง bag (e.g. a plastic bag)
See in Dictionaryครับ polite word for men
See in Dictionary

ถุง bag (e.g. a plastic bag)
See in Dictionary means a bag. ใส่ put something in; put on (e.g. clothes)
See in Dictionary means to add or put something. ไม่ no, not
See in Dictionary means "no" or "not". Thailand is flooded with plastic bags. Major shops stopped offering bags in 2020, but especially street food vendors still pack everything in bags. Please help reduce waste and reject bags whenever possible. 

Women say: ห้องน้ำ toilet, bathroom
See in Dictionaryอยู่ที่ to be at
See in Dictionaryไหน where; which
See in Dictionaryค่ะ polite word for women
See in Dictionary
Men say: ห้องน้ำ toilet, bathroom
See in Dictionaryอยู่ที่ to be at
See in Dictionaryไหน where; which
See in Dictionaryครับ polite word for men
See in Dictionary

น้ำ water; liquid
See in Dictionary is water, so you are literally asking "water-room is where?"

Women say: ขอโทษ excuse me; sorry
See in Dictionaryค่ะ polite word for women
See in Dictionary
Men say: ขอโทษ excuse me; sorry
See in Dictionaryครับ polite word for men
See in Dictionary

You normally say this when you did something wrong. It literally translates to "please punish".

Women say: ไม่เป็นไร That's all right / Never mind
See in Dictionaryค่ะ polite word for women
See in Dictionary (mai ben rai ka)
Men say: ไม่เป็นไร That's all right / Never mind
See in Dictionary ครับ polite word for men
See in Dictionary (mai ben rai krab)

Literally "Not is anything". You use this when people offer you goods and don't want them. You can also use it if someone bumps into you and asks if you are ok.