Well, scratching on basses generally gets you that deep bouncy sound, and scratching on melodies usually gets you that grating harsh sound.

So, what do I scratch on to get that high pitch 'squeaky' sound ?

You need a scratch record with samples on it. They usually put that type of sound on there somewhere.

vocals
Get hold of a hip hop/r&b record with an acapella on it (normally a promo) and experiment.

Worth seeking out some battle/sample records too if you want interesting sounds to scratch with- or get crate digging to find some gems