Your best bet here is to take the SIM card out of your previous phone and insert it into your new phone. If it fits, and your phone is getting service, you don’t even need to let your carrier know about the switch!
If your old SIM doesn’t fit (or this is your first phone) you’ll need to get a Nano SIM from your carrier. They’ll be able to either set you up with a new phone number, or port over your old number if needed.
If you need a new SIM, the next step would be to get in touch with your carrier. Please see your carrier’s instructions below:
When in doubt, visiting your carrier’s nearest retail location to have the SIM activated in person is always your best bet.