Change the layout of the sitepage

Hi, I have difficulties in changing the layout of the page… I hope someone can help me…

So the navigation bar is like this when it is in a maximized window

And when I minimize it, it becomes like this

I tried to fix it using css but then it doesn’t work so I started to think that maybe I need to change it in the .php file itself.
Please help me, I am not a programmer as I just started learning it in the class…

Thanks before!

The CSS Box Model is a good place to start in understanding the boundaries of HTML elements. The entire course at w3schools is excellent and may help to guide you.