Fixed header menu when scrolling

@cjcomms: For this you’ll need to add some Custom CSS and Custom scripts. I recommend you to use Catch Web Tool’s Webmaster Tool for add Custom Scripts.
https://wordpress.org/plugins/catch-web-tools/.
1. Adding Script
-Download and install the plugin and activate it. Go to Dashboard=> Catch Web Tools=> Webmasters.
– Check Check to enable opion in Enable Webmaster Module
– Add the following script in Header and Footer Scripts on bottom text box and click save.

<script>
jQuery(document).ready(function($){
	$(function () {
		$(window).scroll(function () {
			if ($(this).scrollTop() > 280) {
				$('nav.nav-primary.search-enabled').addClass('sticky');
			} else {
				if( $('nav.nav-primary.search-enabled.sticky').length > 0 ){
					$('nav.nav-primary.search-enabled').removeClass('sticky');
				}
			}
		});
	});
});
</script>

2. Add Custom CSS:
– Go to Dashboard=> Appearance=> Customize=> Theme Options=> Custom CSS box and add the following CSS:

.admin-bar .nav-primary.search-enabled.sticky {
    position: fixed;
    top: 32px;
    width: 100%;
}

.nav-primary.search-enabled.sticky {
    position: fixed;
    top: 0;
    width: 100%;
}

Regards,
Mahesh

@cjcomms: Go to Dashboard=> Appearance=> Customize=> Theme Options=> Custom CSS box and add the following CSS:
1. Remove the white space above and below the logo in header

/* Remove the white space above and below the logo in header */
#masthead {
    padding-top: 0;
    padding-bottom: 0;
}
#site-branding {
    padding: 0;
}

2. How many pixels wide should the artwork be
Since the wrapper div is 1200px wide, you’ll need logo of width 1200 pixels

3. Remove the white space above and below the side bar
I am afraid this is not possible.

Regards,
Mahesh