Things you need:
External hard drive ($100+)
Organization
Built in recovery program*

~ Always back up your files.

This includes, but not limited to, pictures, videos, programs, documents. Basically everything that you find useful; use on a frequent basis.

~ Delete unnecessary files on the computer.

Saving files to your external hard drive and then deleting them on your computer saves a lot of space. Deleting/uninstalling programs that you never use when you first get your computer (or when you reformat it).

~ Be organized with your files.

This is important especially in the hard drive. Place them in appropriate folders. Pictures in a pictures folder. Installation programs in an Installer folder, etc.

~ Remember your settings!

Very important especially if you like to customize your computers settings.

~ Have a solid foundation to take care of your computer.

Virus protection program
Disk defragmenter
Chat program
Firefox

~ Reformat every season.

It's recommended that you reformat every season or so; when your computer gets infected; when programs become wonky; or if you just need to organize files once again.

* Built in recovery program

A built in recovery program such as Phoenix FirstWare Recover Pro can save your heinie many times, even when your computer is drowned in viruses.

You can also use the system recovery disks that came with your PC to restore everything to factory settings. However, a lot of the newer PCs no longer have these disks included and instead have a recovery partition.

But such recoveries don't include extra software you added AFTER you first used your PC. So that's where what is said above comes in handy.