I can't find my SIM card on my iphone 4

There is no user-accessible SIM card on that model as I said above.

The SIM card tray is on the right side of the phone. Eject it with a paper clip or SIM unlock tool. If you do not see it, then you probably have a Verizon (CDMA) model which does not have an accessible SIM card.

A CDMA iPhone 4 does not have a SIM card slot. It is strictly a CDMA-only phone. The iPhone 4s included both CDMA and GSM radios into one device, but it was the first iPhone to do so.