Firstly your games - you should have been given a licence key or activation code or whatever the company decides to call it. As long as you have those, you should be able to reinstall them again.
Back up your personal files - that means your documents, music, photos and videos.
You can do this with cds, dvds, usb flash drive or external hard drive very easily.
Open the folder you wish to copy - eg My Documents - click edit - select all
Now click 'copy the selected items' in the left hand pane and choose the destination.
You may want to back up your email contacts and emails if they are not web based, and your browser bookmarks (favourites)