It looks like you're asking what questions to ask, which is great for a scientist! You seem to know already that you'll need to gather information on how high the ball actually bounces, which is good, so I'll go on to other things.
One thing you may want to look at is the angle of your ramp (how much it is tilted) - you may want to see if this makes a difference in how high the ball bounces.
You may also want to look at how flat your basketball is - imagine a nice ball that has a lot of air in it, and an old ball that has been in your garage for a few years and is sort of squashed - I bet you can imagine that there would be a difference in the way they bounce.
You might want to gather information on if there is any effect if you give the ball a push at the beginning, or just let it roll on your own.
These are all suggestions, what I recommend is to pick a few and see how they affect your experiment. Good luck!
(republished on 07/13/06)