First of all, in roma-ji, the correct is DOUMO and ARIGATOU. (We don't have "L" for roma-ji and in hiragana doumo is どうも and arigatou is ありがとう.) どうも and ありがとう are incomparable. ありがとう is "thank you". どうも is an adverb and it has many meanings/ nuance depending on the sentence or the word which is used with it, in this case, it's like "very much" of "thank you very much" though it's not the literal translation. (It's not the informal form by itself.) The basic and common translations are: casual / informal form ありがとう = Thank you / Thanks どうもありがとう = Thank you very much / so much / Thanks a lot formal form ありがとうございます = Thank you どうもありがとうございます = Thank you very much / so much I hope this helps you.

I beleive it is pronounced 'Arigatou' or ありがとう。And if you want to make arigatou more formal add 'gozaimasu', with the 'u' silent. So to be very formal you can say ありがとうございます. Domo, I think is informal, so if you were with a friend. I may be wrong so consult other answers before me.