How do I fix the header at the top of the page?

Go to Theme Options > CSS Settings and add:

#header {
position: fixed;
width: 1200px;
}

@media only screen and (min-width: 1024px) and (max-width: 1199px) {
	#header {
	width: 1000px;
	}
}

@media only screen and (min-width: 960px) and (max-width: 1023px) {
	#header {
	width: 900px;
	}
}

@media only screen and (min-width: 768px) and (max-width: 959px) {
	#header {
	width: 748px;
	}
}

@media only screen and (max-width: 767px) {
	#header {
	width: 100%;
	}
}

#content-wrapper {
margin-top: 70px;
}