Help, my phone keeps turning off randomly please help?

Hi I am a teen and very stressed about this, school is starting soon and my phone keeps randomly turning off when I have a fair amount of battery left, I understand that you may need to reboot the phone and all that jazz but is there a more simpler way to do this? I see that many of the people on this site have the same problem my phone is a iPhone 3GS and any help is greatly appreciated THANK YOU

-MAHPHONEISDUMB