The acceleration of the ball is negative by widely acceptable convention. If we treat direction up as positive, and direction down as negative, then a falling ball will have both negative velocity and negative acceleration. A ball that is projected upward will have positive velocity, and negative acceleration.
Start getting accustomed to using direction up as positive and direction down as negative. Colleges, high schools, and text books use this convention.
You need a book that have problems of this nature and strategies that can be used to solve them. I recommend "Score High in Physics" which is available on the Amazon website. It is a low cost ebook that can be readily downloaded to ipad, kindle, computer, etc.