Best to provide code for future reference but use a CSS selector to select the element you want to stick at the top of the page. Using position: fixed with property top:0 essentially takes it out of the flow of the DOM.
Lets say:
html
<div class="navbar">
 <ul>
  <li>Item</li>
  <li>Item</li>
  <li>Item</li>
 </ul>
</div>
css
.navbar {
  position: fixed;
  top: 0;
  width: 100%;
  overflow: hidden;
}
Bootstrap is used and tested CSS framework that helps reduce development time, and has documentation for numerous scenarios, one being fixed nav. Check it out https://v4-alpha.getbootstrap.com/examples/navbar-top-fixed/
solved How can i make static header for a web page? [closed]