I am asuming this is a road bike.
Fittemd to your carbon steerer is a wedge which will be loose. You needs to remove the top cap with a 4 or 5 mm Allen key. That bolt will fit to the wedge which uses a 6 or 8mm Allen key.
To setup the wedge loosen the bolt and Oush the wedge fully into the steerer tube and retighten the wedge.
Then refit the top cap, and just before you tighten the top cap undo the stem bolts, then tighten the top cap bolt, followed by the two stem bolts on the side.
Note the top cap bolt only needs to be pinched tight just till the rocking in your headset is gone.