Easy! All you need to do is plug your iPhone into your laptop / PC using the cable that you use to charge your device, open up iTunes (latest version) and then wait for it to sync. Once it's synced, click the little iPhone in the top right hand corner, and find where it says "back up". Follow the instructions to what it says when you've clicked it and boom! You've backed up your iPhone. Do not get impatient if it takes a long time, this is likely to happen if you have a lot to backup. Hope I helped.
- Emma :)