The sound [ θ ] in the word "thank" is found in I (initial syllable). It is the unvoiced partner of the" voiced th" /ð/ (you make sound with the vocal cords) that we can find for example in the words "the, father".
In both cases the tip of the tongue is placed behind the top front teeth. Other examples of [ θ ] sound can be seen in "three", "think", "thin".