Help! My navigation bar wont stay centred, footer doesn't work + 1 more problems

I’m a total newbie to coding etc, so have been making changes in a try it and see fashion.

I’ve moved the navigation bar to the header and changed some script when the menu pages weren’t showing on the same line. I then decided to take one of the pages out so that they all showed on the same line, however, now the navigation bar won’t centre and starts on the left.

How to I change it so that is floats in the middle?

Also, since I move the navigation script to the header my footer doesn’t work, why?

And, I’ve inserted images on my pages, however, I want to reduce the spacing between the bottom of the page title bar and the top of the image, how can I do this.

Please help, I’m veeeeeeeeeeeeeery frustrated.

Tanya