"Thank you so much" - 대단히 감사합니다 (a bit formal; 대단히 = very much) - 정말 감사합니다 [고마워요] (less formal; 정말 = really) - 너무 감사해요 [고마워요] (colloquial; 너무 = too, excessive, often used to mean "very")

If you're looking for simple Korean phrases, you can start with the Omniglot site: http://www.omniglot.com/language/phrases/korean.php There's also a vlog about thanking in Korean: https://www.youtube.com/watch?v=POz6web6h_4

감사합니다/감사드립나다 고마워요

thank you is ( without a korean keyboard)= kam sa ham ni da (pronunciation). I don't know about adding very much.