You're going to have to give us some information to work with, here.
What version of the phone (Verizon, AT&T, international....)?
How long have you had the phone? Did this just start happening, or has been a problem since the phone was new?
What (if any) apps do you have installed? What apps or configuration changes did you make before it started freezing?
Does it typically freeze when you're in a certain app? Of does it freeze even if you just turn the phone on and let it sit there, doing nothing?
Does it freeze when you're connected to WiFi, or cellular data, or both?
Describe exactly what happens when it freezes. Is it touch input that stops working, or does whatever is happening on the screen stop working? If someone calls you while the phone is frozen, does it ring? Can you answer?
Do you have an SD card inserted? A SIM card?
What do you do to restart it?
Any other details you can give us may make it possible to help you. But right now we simply have nothing to go on.