Today, at long last, iOS 7 is available to the public!
Before you download and install the update Apple recommends that you backup your iPhone first. If your backup is larger than the 5GB of free iCloud storage you should create a local backup on your computer. You do not want to be in the unlikely event of a bad iOS update and all your pictures and music lost forever.
Here are instructions on how to backup your iPhone and install iOS 7
Apple iOS product and marketing information: http://www.apple.com/ios/