:first-child

/* Selects any <p> element that is the first child of its parent element */
p:first-child {
  color: lime;
  background-color: black;
}

:first-child

<div>
  <p>This p is styled, as it is a p AND the first
     child of its parent.</p>
  <p>This p is not styled; it is not the first child!</p>
</div>

<div>
  <h2>This h2 is not styled; it is not a p!</h2>
  <p>This p is not styled; it is not the first child!</p>
</div>

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

<ul>
  <li>List 1</li>
  <li>List 2</li>
  <li>List 3</li>
</ul>

li{
  color: blue;
}

li:first-child {
  color: green;
}