Edit in JSFiddle 

document.body.addEventListener("click", function(event) {
  if (event.target.nodeName === 'H1') {
    event.target.nextElementSibling.style.display = "inline";
  }
});

<body>
  <h1>Nagłówek 1</h1>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nulla venenatis dui dolor1.</p>
  <h1>Nagłówek 2</h1>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nulla venenatis dui dolor2.</p>
  <h1>Nagłówek 3</h1>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nulla venenatis dui dolor3.</p>
  <h1>Nagłówek 4</h1>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nulla venenatis dui dolor4.</p>
</body>

$black: #000;
$gray: #303030;
body {
  font-family: 'Lato', sans-serif;
}

h1 {
  color: $black;
  font-size: 20pt;
}

p {
  color: $gray;
  display: none;
  font-size: 14pt;
}