The position of the SIM card holder depends on your phone model. It might be placed under the battery or on the side of the phone. Before inserting/removing your SIM card, turn off your phone and make sure no charger is connected to the phone. Insert your SIM card as shown in the illustration that you find close to the SIM card holder.
Note: Without a SIM card, you can’t make or receive calls. In some countries you can make emergency calls without a SIM card.