Unless you've done something strange, it'll be the internal disk. 80 GB on the iLamp, if I recall. When was the last time you emptied the trash?
To see how much space you have, right-click on the disk on the desktop (likely to be called 'Macintosh HD') and select 'get info'. If you're one of those one-button oddballs, click and do command-I.
Help me Mac Users Group, you're my only hopeNo, not really. Late one night, Deep13 was walking down the street when he came across a man on hands and knees, searching the pavement. "What's wrong?", asked Deep13, being a helpful sort of chap. "I've lost my car keys!", said the man, in some desperation. "I can't get home without them!". So Deep13 helped him search. Ten minutes later, still out of luck, our hero asked "where exactly where you when you dropped them?". "About thirty yards down the road", said the guy. "I'm searching here because there's a street light".
Much as I like DU, it's not the best place for Mac technical questions, since Macs are pretty tangential to the place. The best forums for Mac questions are those where the most Mac users congregate.
Apple's own 'discussions' site is the obvious example.
Oh, it's the kind of i-Mac with the dome CPU and the flat screen on an adjustable neck. It came out a few years ago. It is OS X "tiger" if I remember rightiMac G4 - that was my first Mac. Nice machine. They shipped with at least two versions of OS X in their lifetime. If you're not sure, click on the Apple menu and select 'about this Mac'. It'll tell you the OS version:
10.2 = Jaguar
10.3 = Panther
10.4 = Tiger
10.5 = Leopard (but there's no way you could have this without knowing you have it!)