are you trying to make something that uses Apple wallet?
if so, I would start at about 15 minutes into this - https://developer.apple.com/wwdc18/720 session.
if you are trying to do something within your app, you can’t do NFC passes without Apple Wallet, but you can make a QR code. here is a example function that generates a QR code from a string:
func generateQRCode(from string: String) -> UIImage? {
		let data = string.data(using: String.Encoding.ascii)
		if let filter = CIFilter(name: "CIQRCodeGenerator") {
				filter.setValue(data, forKey: "inputMessage")
				let transform = CGAffineTransform(scaleX: 3, y: 3)
				if let output = filter.outputImage?.transformed(by: transform) {
						return UIImage(ciImage: output)
				}
		}
		return nil
}
after you have that function, you use it like this:
let image = generateQRCode(from: "i am going to be so helpful to st0321!")
that generates a QR code with the text "i am going to be so helpful to st0321!" and gives it to you as a UIImage.