How to create header with an image and a menu?

I have a very old webpage that I want to update. (establish in 2007, and updated in 2015)

The original theme is a child of twenty twelve. Main reason for creating a child theme 10 years ago was, to create a header with only a small image, a 3 column front page, to hide login, get rid of comments and to establish a footer divided into 4 columns (Widgets) .

When I change theme to TT3 then edit-possibility of the menu disappear and the theme-editor ask if i want to create a menu.

I do not understand this at all? Why do I have to make new Menu? How do I make a menu?

If my version of TT3 is broken, How do I restart?

I have been working with WP since version 1.5 / 2005, and do not understand why it suddenly have to be so complicated..

I have established a mirror of the page. the link is to the original page.

The page I need help with: [log in to see the link]