kumopsinida  qigu.
I am not  a native .
welcome  Korean friends  to apply the right answer!

<img src="http://www.italki.com/fckeditor/editor/images/smiley/msn/regular_smile.gif" alt="" />
Informal way-------->고맙다,친구야!!!! or 너무 고마워!

친구,너무(매우,진짜) 감사합니다(고맙습니다)  .

Okay . Hahaaaha . <img src="http://www.italki.com/fckeditor/editor/images/smiley/msn/angel_smile.gif" alt="" />

친구, 진짜 고마워! Thats what i would say, but i'm not a native haha ^-^