npm install gives error “can’t find a package.json file”

You don’t say what module you want to install – hence npm looks for a file package.json which describes your dependencies, and obviously this file is missing.

So either you have to explicitly tell npm which module to install, e.g.

npm install express

or

npm install -g express-generator

or you have to add a package.json file and register your modules here. The easiest way to get such a file is to let npm create one by running

npm init

and then add what you need. Please note that this does only work for locally installed modules, not for global ones.

A simple example might look like this:

{
  "name": "myapp",
  "version": "0.0.1",
  "dependencies": {
    "express": "4.0.0"
  }
}

or something like that. For more info on the package.json file see its official documentation and this interactive guide.