The best way to go about this for a home computer is to first consider what needs to be backed up. Generally you only need to backup the files and documents that you have created, but figuring out how much of that you have can be tricky.
The easier way would be to find out how large your hard drive is, say 500Gb, then purchase a USB external hard drive of at least that size. Some of the external drives come with some backup software, otherwise the more recent version of Windows have backup software as part of the package.
Using the backup software you can typically choose to backup your My Documents files and folders (this at a minimum), all files and folders on the hard drive or a selection somewhere in between.
Obviously the more data you backup the longer it will take so if a regular timed option is available then select a time when the computer is on but not in use for an hour or so.
Don't forget that programs backed up will often not worked if restored but some programs, especially accounting packages, do not store the files you have created in the normal My Documents structure so a full backup may be required to grab all these files placed into weird locations by programs not storing them in a standard way.
The final process is making sure your backups are actually doing that, it would be the worst feeling in the world to find out your safe files don't exist.
