get one's brain in gear

get one's brain in gear (make oneself start thinking clearly/effectively) {informal} — собраться с мыслями; привести мысли в порядок; сосредоточиться (контекст.)

Example: I've got to get my brain in gear for the exam this afternoon.

See also
[put on one's thinking cap]
[get one's act together]
[ломать голову]