I dont know how to buy sim card

Look for / visit a carrier, take a subscription, postpaid or prepaid card.

Check whether the SIM card is suitable for your iPhone.

Hopefully you have an carrier unlocked iPhone.

See this Apple article: How to unlock your iPhone for use with a different carrier

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

• Apple can't unlock your iPhone for use with a different carrier. Only your carrier can unlock your iPhone.

You need to contact your Carrier.

I would like to change sim card

Why?

If your SIM card is not working: remove and reinsert the SIM card.

If it doesn't help contact your carrier, maybe the SIM card is damaged, ask for a replaced (new) card. Normally it is free.

Also ask to check if the SIM tray is bent, as even a tiny misalignment can cause the SIM card to not fully connect with the internal contacts in your iPhone.

If you buy or change your SIM card, that means you change your phone number.

So you need to to change your trusted phone number and others.

if you'd like to buy a sim card, go to a internet provider, and simply ask them.

If you have a subscription on you're current sim card, then go to you're current sim card provider and ask them to cancel it.

If what you want is a guide on how to switch sim cards, then here it is : Remove or switch the SIM card in your iPhone - Apple Support