How to activate a sim card

It should be done automatically by the phone company/carrier that issued it. If this isn't happening, read these guides:

If you can't activate your iPhone - Apple Support - "If your iPhone can't be activated or an alert says the activation server is unavailable or the SIM card is unsupported, learn what to do." Click here and read --> https://support.apple.com/HT201407

If you get an alert about the SIM card in your iPhone - Apple Support - "If you get an alert that says Invalid SIM or No SIM Card installed, follow these steps." - https://support.apple.com/HT201420