How large is your internal hard drive, how full is it, and what do you use it for?
Do you have lots of music and documents that you'll just need to back up once and then forget about? Or do you work with lots of large files that will require regular large backups?
The most simple solution is to buy an external hard drive as others have suggested. For less than $100 you can buy a drive large enough to back up your current system, but it might be wise to buy a larger drive in order to ensure room for longer term backups.
If you've never used Time Machine before, it's fairly simple. You plug in the hard drive and the system will ask you if you want to use it as a time machine backup drive. The system will begin making an initial back up which will take much longer than you would expect it to. (Little files that wouldn't normally take terribly long to transfer from one drive to another each have to be moved and catalogued individually.) After that, just plug in the external drive as often as you'd like to perform a back up. Backups are performed automattically on a regular basis, or you can tell the system to go ahead and make a backup manually at any given time.