Some Styling Questions

I have a couple of questions that are based on styling the site. http://coloreddiamondguide.com/education/color/

1) [heading]Our Mission[/heading]
Should I use the heading tag above or should i manually use the h2 tag for a sub heading? Are they the same thing or one is better for SEO than the other?

2) I would like to increase the font size of the content. Which lines of CSS should I change?

3) In the middle of the page is a testimonial box. How can I shift it in line to the middle and that the text does not wrap around the box?

Sorry for the constant bothering. I am very new to css stylings and am doing my best to learn to do everything by myself. Once I get over the initial hurdle to customize some pages I should be fine.

Thanks again!

Paul