My Startup Disk Is Full

I keep getting this message: "YOUR STARTUP DISK IS ALMOST FULL. You need to make more space available on your startup disk by deleting files. [] do not warn me about this disk again. []ok"
I've tried deleting files and music on my iTunes (I think that's the problem) but I still keep getting this message! Is there something I could do that I don't know about? Or is there something i can buy to give me more space?
please help!
I have OS 10.3.9, its a G4, it has 640 MB of memory

One thing to do is when deleting things, make sure you are emptying the trash otherwise your only technically moving the files about.

Also how are you deleting songs from iTunes? You might just be deleting the reference to the song in the library and not the actual MP3. Use spotlight search (top right hand corner) to search for things you no longer use like old applications etc and then sort things out that way

~~ TS ~~

Monolingual for mac allows you to delete languages from your system that you don't use. This can save a few gigs.

here is the website to download the app http://monolingual.sourceforge.net/

thank you both for your help! Tendersurrender, I cant seem to find spotlight search, is there another place it could be?

You dont have it unfortunatly, it was first introduced in 10.4 (Tiger)

Onyx is a very useful tool that will allow you to delete cache and temporary files (make sure you download the Panther, not the Tiger version). Sometimes rebooting will free up a gig or two of space.