How to keep the header fixed

http://www.asl19.org

https://asl19.org/

Thanks for your help and sorry for my late response
But I still can’t fix it
as soon as I change the relative to fix, the header disappear and just the navigation stays.
this is my Header_container at my CSS

*/

#header-container { position: relative;
background: url(images/header-bg-1.jpg) top center repeat-x #272625;
color: #FFF;
border-bottom:3px solid #2f2d2b;}
#logo {}
#site-logo { text-align: center; }
#site-logo img { max-width: none; }
#site-description {
margin-top: 0px;
text-align: left;
font-size: 11px;
line-height:18px;
font-style:italic;}
.header {
padding:0px 0;}
.header-1 {position:relative;}
.header-1 #site-description {
text-align:center;}
.header-2 {position:relative;}
.header-2 #navigation-container {
position:absolute;
bottom:0px;
right:0;
text-align:right;}
.header-3 {padding:0px 0;}
.top-sidebar-area {text-align: right;}
.top-sidebar-area div {float: right;}