How do I install a SIM card on my iPhone?

See this:

For a nanoSIM:

Remove or switch the SIM card in your iPhone - Apple Support

For an eSIM:

Set up eSIM on iPhone - Apple Support

By using a tool provided by apple or your mobile carrier and insert it on the side small hole. The track will come out and put it exactly how it looks if this is your first time I recommend having someone else show you. Thank you for choosing apple

Your iPhone is carrier locked. Apple does not unlock phones. You will need to contact the carrier. They are the only ones that can unlock it for you.

Writing an effective Apple Support Commun… - Apple Community