npm init
This will ask you a bunch of questions, and then write a package.json for you.
It attempts to make reasonable guesses about what you want things to be set to, and then writes a package.json file with the options you've selected.
If you already have a package.json file, it'll read that first, and default to the options in there.
It is strictly additive, so it does not delete options from your package.json without a really good reason to do so.
npm-json(1)