How to partition a drive in X?

I was by someone that they patitioned their FP iMac harddrive into 2. I attempted to do it. I went to applications-utilities-disk utilities- I saw where I select my internal drive. I see where there is a partition selection. But it is greyed out and I can't do anything. It looks like it is locked. Do I need to go through my own administrator's password somewhere or what to allow me to partition my internal hard drive?

I would appreciate it if someone could give me the exact steps to do this if it can be done.
Thank you,
Craig