If a ball is thrown straight up, where is its acceleration the
greatest?
where does the velocity = 0?

When a ball is thrown up acceleration is same throughout the journey and velocity is zero at its maximum height.

Explanation:

The movement of a ball thrown up is completely under the influence of gravity. Thus the only acceleration it can have is the acceleration due to gravity . Hence,  at every point in the upward and downward journey its acceleration will be equal to g.

At the maximum height the velocity of the ball will be zero since it is momentarily at rest.