A full system image that would allow you to restore the computer back to the exact state it's in now, or do you just want to ensure you have a copy of all your important files?
If you want to do a full system image backup, you'll need dedicated backup software which will allow you to create boot media first. You won't be able to do a system backup of a hard disk from which your machine is currently running.
But, assuming like most people you save everything in your Documents folder, you can simply close all applications, plug the HD in, and copy and paste...