How to a full factory reset on the Iphone 3gs?

My iphone 3gs, i've had for a few months now is having a few problems, it keeps turning off randomly just listening to music or if i'm playing on an App. I asked what i should do at the O2 shop, they said try getting it back to factory settings? So, my question is, how do i get it to factory Settings and any idea what this problem is?

Plug your phone in your computer and open itunes. You will see a bar and right click your iphone name. If you want to save your files click backup. Then after backup right click your iphone name and click restore.

Just plug into iTunes & you will see the option to Restore. Click that & iTunes will handle the rest. Of course back up the phone before as doing a restore wipes everything from it.

You have 2 choices. Choice 1 connect it to itunes and do back up first. Then press retore it will be right infront of you
Choice 2 do back up to your itunes as well and then disconnect cable from pc and go to the phone's menu settings general reset then erase all contents
P.S: if you have itunes 11 to get the old view tap on Control button and letter S
Back up doesn't include media files so you have to go to your computer/my computer to copy pictures from the iphone file then paste it on your pc. If you have a mac just open iphoto and press on import all pictures
Once done right clivk on your device's name under yoyr device and click transfer purchases so it will transfer all stuff you have purchased… Just in case. To back up you will follow the last line but instead select backup… Good luck