You’re going to have to detect the scroll position of your page and the (vertical) position of the sections on your page, and then based on that, give the corresponding menu item in your menu a specific class.
http://api.jquery.com/scrollTop/
http://api.jquery.com/position/
Something like this (not tested):
if ( $('body').scrollTop() > $('.section-a').position.top ) {
$('.menu-items').removeClass('selected');
$('.menu-a').addClass('selected');
}
if ( $('body').scrollTop() > $('.section-b').position.top ) {
$('.menu-items').removeClass('selected');
$('.menu-b').addClass('selected');
}
if ( $('body').scrollTop() > $('.section-c').position.top ) {
$('.menu-items').removeClass('selected');
$('.menu-c').addClass('selected');
}