Hopefully I can help
First of all I am going to assume that you have some kind of maximum velocity constant for your ball.
The vector you want would probably be...
dx = (power/100)*Maximum_Velocity*cos(angle)
dy = (power/100)*Maximum_Velocity*sin(angle)
That would be the velocity vector and you would add it to the position vector
Tell me if this works. I used to program games so I might be able to help if you need anything else.