That "clunk" sound or "I need longer fingers&

I only just bought a guitar 3 days ago and of course also have a problem with getting my fingers in the proper position, but i also seem to have a problem with the position of my whole hand and wrist. When i try to play chords such as the "Dadd6add9" in the "easiest song" horse with no name, i feel like i have to really twist my hand to get the clear sound, and it usually results in my thumb pointing almost down towards the body of the guitar and very acheing wrist. Any help would be much appreciated.
Thanks
Ant