first choose which router you really want to use:
1. app router (app/)
2. pages router (pages/)

The app router is newer and easier to learn so I tell you how you do it via the app router:
inside your app/ folder, create a folder something and inside that folder, create a file called page.tsx. Then add the following to the file:

export default async function Page({}: {}) {
  return <p>Hello World!</p>
}

Delete the pages/ folder and it should work correctly. If needed create a new project via: npx create-next-app@latest and choose that you want to use the app router