Cover the Screen
The first thing you should do in case of a broken iPhone 3GS screen is to cover it with a scratch resistant film, such as the ScratchStopper for Artwizz which is specifically designed for iOS apps. This will prevent the screen from further damage as well as help you to avoid any contact with the broken glass.
Get the Parts
You will need a new screen with adhesive to fix it in place. Buy the iPhone 3G Digitizer and front glass that comes with an adhesive kit. Purchase this yourself from a repair store instead of ordering via phone or the internet. If you want to collect the more details about the broken screen of iPhone repair then you may visit http://www.ittekniker.no/iphone-reparasjon.html online.
Ordering will cost you about $119 on these materials whereas purchasing from a physical outlet costs almost half of this amount. Considering that official iOS repair will cost you $199, it will be better for you to avoid ordering to make substantial savings, otherwise get it repaired from the App Store itself.
You will also need to something to open your iPhone 3GS, such as a case opener. Finally, you need a screwdriver with a fine point to open the tiny screws. A number 0000 Philips head is preferable for this purpose.
Note: Since you are working with small tools on a small machine, you need to be a slow as possible in your working in order to accomplish the task without any complications.
Unbolt the Casing
Turn off the iPhone 3GS and remove the SIM card. Now remove the 2 screws on the bottom. This will take some effort and you will also need a tool to separate the metal frame. When pulling up the screen, be as careful as possible, because it is connected with cables.