Help pls

I am actually very confused. So I got AI to code in a POST request to /api ,so front-end is supposedly done. It tells me to do this: Create a Vercel Serverless Function In your repo, add a new file: /api/x.js This is where you call OpenAI. Your client will call /api/x. I have no idea how to do this. Where do I click in Vercel for this... But I have added the environmental variable of the API key. So how do I do the serverless function?