My Sticky Header Cover Content with the Smooth Scrolling Effect

I added a sticky header and a smooth scrolling effect, and I cannot figure out how to fix the position so it counts with the header size. The things I have tried disable the sticky header completely.

I have tried to use several different techniques, although I am a newbie and it might be too hard for me to do by myself.

<div id="container">
  <section id="sectionHome">
    <!--Header and Logo-->
    <header id="myHeader">
      <logo>
        <img src="Pictures/Marvel-logo-880x660.crop.png">
      </logo>
    </header>

    <!--The Top Navigation Menu-->
    <div id="mainNav">
      <ul>
        <li class="current"><a href="#">Home</a></li>
        <li><a href="#firstArticle">Characters</a></li>
        <li><a href="#secondArticle">Movies</a></li>
        <li><a href="#thirdArticle">More Info</a></li>
      </ul>
    </div>
  </section>

//Smooth Scrolling in Main Nav
$(document).ready(function() {
  $('#mainNav li a').click(function(e) {

    var targetHref = $(this).attr('href');

    $('html, body').animate({
      scrollTop: $(targetHref).offset().top
    }, 1000);

    e.preventDefault();
  });
});


// Sticky Header
window.onscroll = function() {
  myFunction()
}; // When the user scrolls the page
var header = document.getElementById("sectionHome"); // Get the header and top nav
var sticky = header.offsetTop; // Get the offset position of the navbar
function myFunction() { // Add the sticky class to the header when you reach its scroll position. Remove "sticky" when you leave the scroll position
  if (window.pageYOffset > sticky) {
    header.classList.add("sticky");
  } else {
    header.classList.remove("sticky");
  }
}

This was one thing I tried, but it disabled my sticky header:

$(document).ready(function() {

  var headerHeight = $('header').outerHeight(); // Target your header navigation here

  $('#main-nav li a').click(function(e) {

    var targetHref   = $(this).attr('href');

    $('html, body').animate({
        scrollTop: $(targetHref).offset().top - headerHeight // Add it to the calculation here
    }, 1000);

    e.preventDefault();
  });
});

I thought I could set a value for the total header size and position it that way, although it disables the sticky header. How do I do this properly?

This is my webpage: http://www.student.city.ac.uk/~aczc972

Best regards, Danielle

I have added a sandbox how to do it using jQuery, generally speaking only one addition from my site is that I am checking what is the target e.g. scroll to top page, and if yes, I am running specified code for it:

if (targetHref === "#") {
  $("html, body").animate(
    { scrollTop: 0 },
    "1000"
  );
} else {
  $("html, body").animate({scrollTop: $(targetHref).offset().top},1000);
}

codesandbox.io/s/81l87w26w0

Subtract header height scroll to prevent covering content by header

scrollTop: $(targetHref).offset().top - 180"