Fixed Nav

Hi Danu,
The best way is to use some CSS, make your navigation fixed. Here is a link to start with > http://premium.wpmudev.org/forums/topic/static-header-bar-stays-with-page-scroll

You can add some javascript to fix the menu while scrolling. Something like this > http://designshack.net/articles/javascript/simple-steps-for-creating-a-fixed-navigation-bar/

And if you want to use plugin, you may use this (there are lots of similar plugins available though) >
https://wordpress.org/plugins/sticky-header/