What you're after is a 'sticky navbar/menu'.
The simplest way would be to add the below CSS to your menu/navbar
position:fixed;
top:0px;
That said, for an effect closer to the one you've posted, you'll probably want to look at using some jQuery, e.g.:
$(window).bind('scroll', function() {
if ($(window).scrollTop() > 50) {
$('.menu').addClass('fixed');
}
else {
$('.menu').removeClass('fixed');
}
});
What this does is 'fix' the menu bar to the top of the page once you scroll past a certain point (e.g. 50px) by adding the CSS class 'fixed' to the .menu
element, the fixed class would simply be e.g. the CSS above.
There are some nice examples listed here.