Start out with the old "Rule of Thumb" method, or "fistmele". Make a fist and stick your thumb up (like your giving someone the thumbs up sign), place the bottom of your fist on grip portion of the bow and then measure where the string hits your thumb.
Every bow's brace height is a little different depending on the shooters form, shooting style and arrow choice. Then start shooting, twisting the string to increase or decrease the brace height. It won't that long for you to figure out at what brace height your bow performs the best at.