how to change margin on page??

Hi there,

I’m a beginner at modifying themes. I did create a child theme and now the top margin is way too big on my pages. How would I fix this? I’d also like to removed the horizontal line.

Here is a link to what it looks like:
https://docs.google.com/document/d/1zUKEOZef_RLhZJqv7YUyR4n605bRxeDjzpraq7cU7MU/edit?usp=sharing

Thanks in advanced!