How to make a move with your scrolling?

Question

I want to make a top navigation bar, but I can't get the to move when a user scrolls down on the page. Here is the code I have written so far:

#menu {
  height: 50px;
  width: auto;
  border: 5px solid black;
  background: black;
}

.menus {
  text-decoration: none;
  color: white;
  margin: 20px;
}

<div id="menu">
  <a href="/about" class="menus">About Us</a>
  <a href="/signup" class="menus">Create Account</a>
</div>