How to login into a website with CasperJS?

You will need to use Casper fill() function.

Below is an example which login to Facebook and print out your name after login. Note that you need to put in your username and password:

var casper = require('casper').create({   
    verbose: true, 
    logLevel: 'debug',
    pageSettings: {
         loadImages:  false,         // The WebPage instance used by Casper will
         loadPlugins: false,         // use these settings
         userAgent: 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_5) AppleWebKit/537.4 (KHTML, like Gecko) Chrome/22.0.1229.94 Safari/537.4'
    }
});

// print out all the messages in the headless browser context
casper.on('remote.message', function(msg) {
    this.echo('remote message caught: ' + msg);
});

// print out all the messages in the headless browser context
casper.on("page.error", function(msg, trace) {
    this.echo("Page Error: " + msg, "ERROR");
});

var url = 'http://www.facebook.com/';

casper.start(url, function() {
   console.log("page loaded");
   this.test.assertExists('form#login_form', 'form is found');
   this.fill('form#login_form', { 
        email: '**<put your email here>**', 
        pass:  '**<put your password here>**'
    }, true);
});

casper.thenEvaluate(function(){
   console.log("Page Title " + document.title);
   console.log("Your name is " + document.querySelector('.headerTinymanName').textContent ); 
});

casper.run();