How do you write "Happy New Year"?
สวัสดีปีใหม่ - Happy New Year

I am allergic to certain food types and I am worried about going to Thailand.
We already have a section which will help you with this. The Thai word for "allergic" is "pare".

Men say: "Phom pare....."
Women say: "Di-chun pare..."

http://www.learningthai.com/allergic.html

This page has sound clips so that you can practice speaking and also a link to print the phrases out in Thai.

I usually learn some words in a new language for every country I visit. Do the Thai like people talking Thai or do they prefer English?
When you arrive in Thailand you shouldn't really have a language problem if you stick to the main tourist route. The hotel staff will know "hotel English", the taxi driver will know "taxi English" and the shopkeeper will know "shop English". However, once you start chatting away about something unrelated you might have a problem. Also, once you leave the major cities and start going to places which don't receive many tourists you will again be presented with a few problems. That is why we are here to help!

I think your first stop should be our:

http://www.learningthai.com/useful_phrases.html

At the very least you should learn these words.

Quite often I want to be able to say "I don't believe you" when confronted by touts.
I don't believe you would be 'mai cheua' ไม่เชื่อ or you could also say 'it's not true' which is 'mai jing' ไม่จริง

- Mike

http://www.learningthai.com/sayno.html

How often do I need to use the polite word "khrab"
The rule is simple: you end your sentences with the word "krub" in polite situations and when you talk to someone who is senior to you.

- w-bpf

When do you use the word "Phi" in Thai?
You use the word "Phi" when you call someone who is not much older than you. For example, my younger brother calls me "Phi Gor" and I call him "Nong".

For people who are a lot older than me, I call them uncle or aunt even if they are not my relation. If the man is older than my father, I call him "Lung". If he is younger, I call him "Ah". If the woman is older than my mum, I call her "Ba". If she is younger, I call her "Na"

"Khun" is the polite way of saying "you". We use it in front of people's name like you use Mr. and Mrs. I don't usually use it.

I think it's better for you to use the word "khun" in front of someone's name.

- Nattawud

We are wondering how much Thai we would need to learn in order to make our trip more successful.
You might want to start off with a phrasebook on Thai language. This should include basic phrases like ordering food, getting accommodation and shopping etc...

Thai Phrasebook by Lonely Planet is good. It has sections on basic Thai grammar, Meeting people, Getting around and also includes a Thai-English and English-Thai mini dictionary. Quite handy and it should fit in your pouch nicely.

Unless you're going to non-tourist destination or rural places, there're bound to be someone who is able to speak English. Just relax and have fun.

- Yeows