Must Have Lenses for a Cropped Sensor Canon?

I'm new to the DSLR world and while I don't exactly have the finances to seriously invest in lenses right now I do understand just how important they really are, but I don't have the best idea of which lenses to look into once I can afford to. So I thought I'd ask the best what lenses give you the real bang for your buck with a cropped sensor Canon body, specifically in my case the 600D. So, thoughts?