1 - Close all applications running in the background.
2 - Reboot your iphone
Applications often crash if too many applications are running in the background causing the flash memory to be over loaded. By default the ipod, email and safari (the internet browser) apps are running in the background after you have used them. These can both be closed fully by returning to the application and holding down the bottom round square button for six seconds, after this time you will be returned to the desktop or application view.
So try closing the ipod, email and safari applications in turn using the six second button hold as explained above. If that does work reboot your iPhone by pressing and holding the sleep/wake button, then a red slider appears on the screen which can be used to completely shut down the phone. Once off press the sleep/wake button again to re start the iphone.