CSS can control how an element looks, and it some cases, how it behaves, but it won’t change what type of element it is. For example you can’t make a heading element into a paragraph or vice versa with CSS.
If you want an element to take on a certain appearance (larger text, a different color, etc) you’ll need to find how that element is marked up so you can specify it in your CSS.
The easiest way to do this is just to look at the source of the page you’re working on. Depending on what browser you’re using there should be an option for View -> Page Source or something similar. From there find the element you’re working on and see what markup is wrapped around it, use that to specify your rules to that element and you can make changes to the appears in your CSS file.
If you instead want to change the element to an h1, you’ll need to do that in the template files that generate the page. The easiest way to find out which file generates that bit of the page would be to turn on the ‘Template Path Hints’ option in the admin panel. You can find that by logging in to the admin panel then System -> Configuration, once there select your website from the dropdown menu in the upper left (you’ll probably Main Website) and then click the Developer option on the menu on the left side of the screen. In there you’ll see a section labeled ‘Debug’ including an option for ‘Template Path Hints’, switch that to Yes and save.
When you go back to your site you’ll see paths showing which file generated each piece of the page, find the one generating your heading and track down that file. From there you can make the changes to the actual structure of the page, you may still need to tweak the appearance with CSS to get exactly what you want.
Really it’s up to you which way to go, good luck with it and let me know if I can help.