If you have gone to the trouble to find this page, you deserve some help - but at the moment I am not quite sure what to tell you. I could do with some useful Web page references that we could use for background information; if you find any, please let me know.
The basic experiment is to investigate how the final speed v of the ball changes, as the vertical height h that it falls changes. You might as well suppose, as a preliminary prediction, that v is proportional to h. The symbolic way of showing this is: v µ h. If two things are proportional, then doubling one, doubles the other, and so on. A graph of one against the other will be a straight line through the origin.
When you have got a preliminary set of data, plot v against h. Does it look as though v µ h ?
Consider conservation of energy. As the ball goes down the slope, it gains ½ m v2 of kinetic energy, and loses mgh of gravitational potential energy. This implies that equal increases in height fallen should give equal increases in v2 gained. Try a graph of v2 against h.
Presumably the KE gained should equal the PE lost. Try this on a table of results, or, better, plot a graph of ½ m v2 against mgh; if these are supposed theoretically to be equal, what should the slope of the graph be?
Have at look at this university-level work-sheet.. Or this one. This animation may help you.
Good luck. Email me if you any urgent questions - and you can email draft work for me to comment on, if you wish. (PHT)