ok i jus got this fone on Tuesday,and the internet was workin jus fine, but now it wnt let me use
it, all my apps say sorry try again later, And i would go to my settings and switch it to on, but as soon as i go back it switches it to off by itself. idk wats wrong with it???? is it a iphone problem or is it with my phone company At&t???? can some please help me thank you for your time

The issue about not using a "death grip" on the iPhone (suggested by the previous answer) applies to using the 3G, not Wi-Fi.

Go back to Settings and scroll down. You will see an option to Reset. That should restore your iPhone to the right settings. I had the same problem with my iPod Touch, and that worked perfectly.

Good luck!