Pop in a blank CD, when asked tell it to open in finder, drag whatever files you want to burn over to the mounted CD-R, when finished, choose File -> Burn Disc...
Harddrives tend to fill up much faster than anticipated...
What size disk did you get, and what software did you load already? And how much data did you copy on?
Expect the default software on the current powerbooks to use somewhere between 12 and 18 GB of space. GarageBand and iDVD use 5GB combined, so that's a nice chunk of space right there.