I want to buy a new Iphone 12 from Apple but I have Cricket wireless. What do I need to do?

Check to see if they allow you to bring your own device. Also if you can purchase and bring a phone from Apple, it will need to be unlocked. See this for contacting Cricket and if it contains any FAQ for bringing your own device. https://www.cricketwireless.com/

Use the selection Connect on your own later. This is the unlocked phone.