These two phrases mean basically the same thing. Possible meanings are 1) Habakkuk went to an actual post in the watchtower or 2) this is a metaphor in which Habakkuk speaks of waiting eagerly for Yahweh's response as if he were a watchman waiting at his post for the arrival of a messenger. (See: [[rc://en/ta/man/jit/figs-parallelism]] and [[rc://en/ta/man/jit/figs-metaphor]])
Here the word "turn" refers to returning an answer. Habakkuk considers what answer he will give concerning the things that he has said. (See: [[rc://en/ta/man/jit/figs-idiom]])