<script type="text/javascript" src="jquery-3.1.1.js"></script>
<script>
var num = 120; //number of pixels before modifying styles

$(window).bind('scroll', function() {
    if ($(window).scrollTop() > num) {
        $('nav').addClass('fixed');
    } else {
        $('nav').removeClass('fixed');
    }
});
</script>

<div id="headwrapper">
    <header>
        <img src="Logo.png">
        <h1>IT TECH</h1>
    </header>
</div>
<nav>
    <div id="selector"></div>
    <a class="link1" href="home.htm">
        <p>Home</p>
    </a>
    <a class="link2" href="talen.htm">
        <p>Programmeertalen</p>
    </a>
    <a class="link3" href="computer.htm">
        <p>Computers</p>
    </a>
    <a class="link4" href="richting.htm">
        <p>Richtingen</p>
    </a>
    <a class="link5" href="contact.htm">
        <p>Contact</p>
    </a>
</nav>
<div id="element">
    <div id="slider">
        <div title="Banaan" id="foto1">
            <h1>Welcome to</h1><img src="Logo.png">
            <h1>IT TECH</h1></div>
        <div title="Peren" id="foto2"></div>
        <div title="Kiwi's" id="foto3"></div>
        <div title="Aardbeien" id="foto4"></div>
    </div>
</div>
<article></article>

.fixed ~ #slider {}