Your new mobile phone should come with the SIM card already inserted or it will not work. If you need to insert it yourself.

Inserting Your SIM Card
Carefully remove the SIM card and insert in the base of your phone in the appropriate slot, usually under the battery. Do not bend or scratch your SIM card. Instructions for inserting your SIM card might vary from handset to handset, ensure you check your phone manual.

Procedures for inserting a SIM differ for each handset. Please refer to your handset manual for procedures.