You go online and pick a plan with Mint. You can verify right on the website if your phone will work. If it does, they send you a SIM card (unless you have a very new phone with an eSIM). Doubtful if you've been under contract.
You need a few things to bring your phone number over:
https://www.mintmobile.com/help/can-...o-mint-mobile/
You can do it online without speaking to anyone. Mint does NOT have customer service reps like VZW does. Part of the reason it's so damn cheap.
It's very easy to do. Son is on Mint. Service is great.