When I run:
npm install my-app
The app is installed into node_modules/my-app/...
I also tried
npm install -g my-app
As @dalu said, if you want to have local packages, you'll need a package.json file.
package.json
But to create the package.json file, you will have to initialize npm by running npm init.
npm init
You can then use npm install --save[-dev].
npm install --save[-dev]