In your syle.css from your theme, I changed this:
from 1000px to 900px and got rid of the horzontal scrollbar. 900px seems oddly small but your theme has big margins and padding.
Get Firefox and then the Firefox addon Firebug. You can then click anywhere in your site and see what piece of the css influences that piece of your site and you can even make changes but not save them, so it is safe to practice.
About aligning left of the content: what I see is left aligned so what exactly do you mean?