Hello @kartik,
If you want to put the routes in a separate file, for example routes.js, you can create the routes.js file in this way:
module.exports = function(app){
    app.get('/login', function(req, res){
        res.render('login', {
            title: 'Express Login'
        });
    });
    //other routes..
}
And then you can require it from app.js passing the app object in this way:
require('./routes')(app);
Hope it helps!!
Thank You!