A newbie's problem with making a navigation dropdown

[Wilfred Thompson]

I am Wilfred Thompson and I am a newbie when it comes to web design. My problem is that I am having a difficult time understanding CSS, HTML, and the HTML element Script which is why my navigation dropdown is a broken mess. My aim is to create a navigation dropdown that starts from a small button with three lines that will ultimately drop down the menus Home, Gallery, About, and Contact vertically and then the close symbol will also appear which will hide the menus if clicked. I want the three lined button dropdown navigation to appear only after reaching screen width 1044 pixels and below because I am fine with the menus being shown and in-line with each other side by side from width 1045 pixels and above. How can I fix this? Please help.

My HTML file and CSS file for the website I am talking about is in the attachment below. Please check on it to further understand my point.