How do I activate my phone

If you can't activate your iPhone - Apple Support

Are you swapping SIM cards? If so, your iPhone must be unlocked, or must be locked to the same carrier as the SIM you’re using:

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

If that’s not what you’re seeking to address, what’s the error message you’re seeing, what have you tried, and what happened when you tried it?