The iPhone 3G was manufactured and released by Apple in 2008 and you may have some problems by now with the phone and need to learn how to reset iPhone 3G as part of a troubleshooting process. The 3G is a smart phone that is 2nd generation. There are two different phones: one has storage of 8GB capacity and the other has 16GB capacity. Some of the improvements over the first generation phone were the GPS and the App store that are included as part of the operating system. This phone was discontinued in 2010 and the operating system upgrade was not included in the new 4.0 OS. If the screen freezes and you cannot get it to force quit you will have to manually reset the phone.
You need to hold down both the sleep and the home buttons simultaneously. Continue holding until the red graphic that indicates shut-down disappears and the screen turns to black. When the logo for Apple appears on the screen the phone has reset and it will restart. If this does not solve your issues with the phone you will have to move to the next step and that is to restore the iPhone.