Edit in JSFiddle

 <div id="slider">
    <img src="https://www.metaslider.com/wp-content/uploads/2014/11/mountains1.jpg" alt="">
  </div>
    <h1>Teste</h1>
    <p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Alias suscipit asperiores a ad mollitia totam natus aliquam accusamus, dicta quidem laudantium quod necessitatibus fuga voluptatum veniam consequatur rerum odio nihil.</p>
#slider img{
  height: 100vh;
  width: 100vw;
}