"Thank you"...

As a verb, "thank you" is two words.

...Thank you for all you do.
...I want to thank you for the chocolates.

As a noun, "thank-you" is hyphenated.

...Please send a thank-you to your grandmother.
...We received several thank-yous in the mail.

As a direct adjective (right in front of the noun), "thank-you" is hyphenated.

...I sent a thank-you note.
...It was a nice thank-you message that she left for me.

Happy punctuating!