increase storage on my Mac Book Pro

I recently bought the Mac Book Pro with Retina display and I love it. However, I thought I was going to save money and simply get the lower storage version (250G). Unfortunately, after transferring everything from my iMac to the Mac Book Pro I find I have less than 1G of storage remaining. Ugh! I really wish the Apple person had walked me through this decision process and pointed this out to me. I've got plenty of external drives with plenty of storage. How do I remove things from the current Mac Book Pro to give me at least a little bit of storage (say 20G) without damaging my Apple installation. I really should have moved up to the bigger Mac. Thanks in advance for your help.

Phil Landry