：first-child

:first-child {
  /* ... */
}

<div>
  <p>This text is selected!</p>
  <p>This text isn't selected.</p>
</div>

<div>
  <h2>This text isn't selected: it's not a `p`.</h2>
  <p>This text isn't selected.</p>
</div>

p:first-child {
  color: lime;
  background-color: black;
  padding: 5px;
}

<ul>
  <li>Item 1</li>
  <li>Item 2</li>
  <li>
    Item 3
    <ul>
      <li>Item 3.1</li>
      <li>Item 3.2</li>
      <li>Item 3.3</li>
    </ul>
  </li>
</ul>

ul li {
  color: blue;
}

ul li:first-child {
  color: red;
  font-weight: bold;
}