How do I install a sim card on my Galaxy Note 5?

Shove an straightened paperclip into the hole and push firmly...the door (and the SIM card) will pop out...

First check that the phone is not locked with a different network. If it is, then you need to unlock. To unlock, I suggest you can approach Prounlocking.com and specify the country,locked network of Samsung Galaxy Note 5.Based on the availability you can get the permanent unlock service along with free instructions.
It uses a nano sim card. so get the appropriate sim size.
Follow instructions from samsung.com/us/support/answer/ANS00043237/ to insert the sim card.