Q: HOW TO ACTIVATE

just follow the instruction

What do you want to activate?

What problems are you having activating your phone? What happens when you try?

The phone is carrier locked. Only the carrier it is locked to can remove the lock. Contact them.

Then, most likely, your phone is locked to a different carrier. You'll need to either activate the phone with the carrier it's locked to or get that carrier to unlock the phone.

You have to get in contact with the carrier it is locked to.  The carrier has to process the unlock request.  It will depend on your contract with the carrier, if it can be unlocked before a long waiting time.

See this support document:

How to unlock your iPhone for use with another carrier - Apple Support

https://support.apple.com/en-us/HT201328