<div id="menu">
  <ul>
    <li><a href="#">Trang chủ</a></li>
    <li><a href="#">Tin tức</a>
      <ul class="sub-menu">
        <li><a href="#">WordPress</a></li>
        <li><a href="#">SEO</a></li>
        <li><a href="#">Hosting</a></li>
      </ul>
    </li>
    <li><a href="#">Sản phẩm</a></li>
    <li><a href="#">Liên hệ</a></li>
  </ul>
</div>

body {
  font-family: sans-serif;
  font-size: 15px;
}
#menu ul {
  background: #8AD385;
  width: 250px;
  padding: 0;
  list-style-type: none;
  text-align: left;
}
#menu li {
  width: auto;
  height: 40px;
  line-height: 40px;
  border-bottom: 1px solid #e8e8e8;
  padding: 0 1em;
}
#menu li a {
  text-decoration: none;
  color: #333;
  font-weight: bold;
  display: block;
}
#menu li:hover {
  background: #CDE2CD;
}

/*==Dropdown Menu==*/
#menu ul li {
  position: relative;
}
#menu .sub-menu {
  position: absolute;
  left: 250px;
  top: 0;
  display: none;
}
#menu li:hover .sub-menu {
  display: block;
}