It's a little technical, and you need to know some about css. Review this page & some great links at the end of the article:
You can check out these tutorials:
A beginners guide to customizing a WordPress site
But basically you create a style.css for your child theme, import the original css file (convered in the Child_Themes codex).
Then any element you want to modify, you list in your new child style.css file, and modify that style:
Regarding your content width: The original style for the #content id looks like this
margin: 0 0 30px 0;
to change the width to 100%, you would place this in your child theme:
Again, this is just a start, lots of trial and error before things fall into place and you get comfortable with custom css styles. Good Luck!!