You then decide which half to take given the functions value at the interval fences.
Your interval is [-3,3] so first of all find f(-3) and f(3). You will notice one result is positive and one negative. This means the root is on the interval.
Now bisect the interval and find the function value for that, f(0) , this bisection gives two new intervals to choose from either [-3,0] or [0,3].
You now have f(-3) = -39, f(3)=45 and f(0)=3 as pick the interval such that either side is still positive and negative. i.e f(-3) = -39 and f(0)=3 means the root is on [-3,0] and not [0,3] , bisect ths new interval again applying the same logic until you converge on the root.