.type and .click selectors in Nightmare.js

I have been working with nightmare.js to run tests using mocha. This is my first time working with these framework/libraries. I am currently creating a test that loads a login page on a web app im working on and attempts to sign in. This is the code I am using as a reference Source: http://ift.tt/2lFjwAH

  describe('Login Page', function () {
describe('given bad data', () => {
  it('should fail', done => {
    nightmare
    .goto('http://ift.tt/2p9v42e')
    .on('page', (type, message) => {
      if (type == 'alert') done()
    })
    .type('.login-email-input', 'notgonnawork')
    .type('.login-password-input', 'invalid password')
    .click('.login-submit')
    .wait(2000)
    .end()
    .then()
    .catch(done)
  })
})

The main question I have about tis code is the .type and .click functions. Reading the nightmare api both of these functions work like so

.type(selector[, text])

Enters the text provided into the selector element. Empty or falsey values provided for text will clear the selector's value..type(selector[, text])

I have tried to implement this functionality using the code below

describe('/ (authentication)', () => {
    it('should pass by logging into the system', done => {
        // testing urls will be `http://localhost:port/path`
        nightmare.goto('http://localhost:4000/login')
            .on('page', (type, message) => {
                if (type === 'alert') done()
            })
            .type('input[id="username"]', 'admin')
            .type('input[id="password"]', 'toetagging')
            .click('button[data-reactid=".0.0.1.3.0"]')
            .wait(2000)
            .end()
            .then()
            .catch(done)
    })
})

My main question is, what exactly is the selector and how do I get it?