Using <div class="space"></div>
before the slider and giving it a height:50px;
in css (the same height as the navigation.)
body {margin: 0; padding:0;}
ul {
background: #000;
color:#fff;
line-height:50px;
margin:0;
padding:0;
position: fixed;
width:100%;
}
ul li {
display:inline-block;
}
.space {
height:50px;
}
.slider {
background:red;
height:350px;
width:600px;
margin:0;
}
<ul>
<li>Home</li>
<li>About</li>
<li>Contact</li>
</ul>
<div class="space"></div>
<div class="slider">
</div>