Well! SIM card is the thing that will help you to run your iPhone. Only after, you insert the SIM card properly, you will able to use your iPhone. Here are some points that will help you if you are unable to insert the SIM screen.
Switch OFF the iPhone first and then take out the SIM.
Now insert the SIM card properly and then turn the iPhone off.
Now check if the SIM card is perfectly adjusted or fit. After that, switch ON your iPhone.