Thank you.

arigatou : thank you, thanks
doumo arigatou : thank you very much

As a phrase to express your gratitude, just learn it as a phrase.

Doumo arigatou. : Thank you very much.
Doumo arigatou gozaimasu : Thank you very much.(polite)

When "gozaimasu" is attached at the end of the phrase, the sentense would be very polite.