There are different ways with different price tags...
Option #1-External Harddrive
The cost the most, but you can get a decent one between $100-$200. It's bascially another harddrive, outside of you computer, that you save your files to. But if anyone were to steal it, tough luck lol.
Option #2-Online Storage
There are free sites and sites that charge you to store your files online. It's pretty simple, you just upload them to their site and into your selected folder. Just do a google search for "online file storing" .
Option#3-Storing Your Files on Disks
This is what I call the lazy person way. Just buy a giant pack of rewritable cd-roms, and everytime you want to save somethings, you save it right to the disk. Its a fast and easy way, but you'll end up with a lot of disks. Hope this helps.