Q: how do i open the sim card holder on i phone se?

Look in the instructions that came with the phone or with the SIM card.

SIM card

If you were given a SIM card to install, install it before setting up iPhone.

Important: A Micro-SIM card (iPhone 4s) or a Nano-SIM card (iPhone 5 and later) is required to use cellular services when connecting to GSM networks and some CDMA networks. iPhone that’s been activated on a CDMA wireless network may also use a SIM card for connecting to a GSM network, primarily for international roaming. Your iPhone is subject to your wireless service provider’s policies, which may include restrictions on switching service providers and roaming, even after conclusion of any required minimum service contract. Contact your wireless service provider for more details. Availability of cellular capabilities depends on the wireless network.

For any future needed information please use the iPhone Help information.

See these instructions:

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