Trying to back up your entire system onto CD would take FOREVER!!! If you could even do it at all. You need an external HDD (Hard Disk Drive). You can pick them up for very sensible money now; a 1tb USB external HDD will cost around £80, smaller capacity drives even less. (
Amazon.co.uk User Recommendation ) So if your computer's HDD is only, say, 250gb you'll only need a 500gb external HDD at most.
How you backup depends on whether you use a Mac or Windows PC, but in both cases it's quite simple and their are dedicated programs within each system to enable you to do this.
Hope this helps.