Xièxie! Thank you!

If you only wish to learn one expression in Chinese, why not go for Thank you! Speaking politely helps a lot, if you encounter with Chinese people or find youself in China one of these days.

Xièxie! Is the word for thank you! in Chinese.
And when returning the compliment, you can say modestly in a Chinese manner bié kèqi, "don't be polite".

There is also a very useful small word qĭng in Chinese language. It means quite the same than "please" in English. If you are offering tea to your friend, and asking him to sit down, just smile and say qĭng.

xièxie 谢 谢 – thank you!
bié kèqi 别 客 气 – don't be polite = you are welcome!
qĭng 请 – please!