There really is no way we can give you good help with just this pick. We would need all of your HTML and CSS. But if I had to guess, I would start by removing the default margins that the browser puts on the
To display your code in here you need to wrap it in triple back ticks. On a line by itself type three back ticks. Then on the first line below the three back ticks paste in your code. Then below your code on a new line type three more back ticks. The back tick on my keyboard is in the upper left just above the Tab key and below the Esc key.
Thanks you so much for your advices, I will try to do it . As a new coding learner , having many to study.