'Thank you' in Mandarin Chinese would be xièxie nĭ or simply say xièxie.
'Thank you' in Cantonese would be dòjeh (saying thank you for a gift) or m̀hgòi (saying thank you for a service).
Learn some of the useful phrases in Mandarin Chinese and Cantonese at Omniglot.
Mandarin is used across the north and southwest of China. With around a billion speakers across the globe, Mandarin is spoken by more people than any other language (including English). It is an official language in Taiwan and Singapore and is commonly used in ex-patriot Chinese communities around the world.
Cantonese is a variety of Chinese spoken in Hong Kong, Macau, southern China, and a number of other places, in traditional characters and Yale Romanization.
Check out some tips to learn Chinese effectively from MandaLingua Shanghai.