You should do the following things:

1. Serve your static files


2. Create an API server that will listen for the requests coming from your frontend app

1. Serve your static files

To serve static files with Express, read this link.

You'll basically add it to your express app:

app.use( express.static( __dirname + '/client' ));

Where '/client' will be the name of the folder with your frontend app files.

2. Create an API server

You can see how to create an API server here.

For the entry point of your application, you should send/render a file.

This could be accomplished with the following code:

app

  .get( '/', function( req, res ) {

    res.sendFile( path.join( __dirname, 'client', 'index.html' ));

  });

This will send a static file every time that a user request a file at the root path of your application.

You can use the asterisk * (wildcard) instead of / too. That symbol meaning that for whatever route requested, you will respond with the same file/action.

More about the responses here.

Sum up

Those are the things that you should seek to build your app.

You can see a simple app with those things implemented here.