A lot of styles are set in the (external files) CSS for your theme - a browser tool like Firebug will help you find those - but don't change any theme files directly - as those changes will get overwritten when the theme is updated. Use a child theme or custom CSS option -
http://codex.wordpress.org/Child_Themes
So, HOW do I make the site FIXED with WHAT codes and WHERE?
It really depends on what you are trying to change - if it involves an existing style, you often need to COPY the existing CSS to the child theme style.css file and make the change there. That file will override the existing CSS (assuming you've done it correctly).
If you are adding a style (i.e. new color for an element), you can add a class to the HTML code in the TEXT editor, and use that class to add the CSS to the child theme style.css file.
Some styling can also be done using the Visual Editor - but again, it depends what you are doing. You might also want to look at some plugins that can give you more options in the Visual Editor - like this one: