as far as i was aware you couldn't shrink an already partitioned drive, instead you'd have to restart from scratch but i wait to be proven wrong.
tbh i think no matter what people say you should partition your drive, even if it proves pointless in terms of performance, learning is still learning and you may one day need to know how to do simple tasks like this which in future would only take 10 seconds, rather then an hour or 2 to do.