Best way to find correct css selector?

[frygurl 0]

Let's say, for instance, I want to create a black header bar that spans the full width of the browser window, like the one on this page. How do I determine the right CSS selector to use when I add custom code? I have Firebug, but so far the things I have tried have not worked. I am hoping that I can get help learning how to fish, so to speak. I want to know how to figure these things out for myself. Thanks in advance!

[frygurl 0]

I think I figured it out!

[Martin Bay 20]

I usually view the Page Source of the page Im working on.

[Danny+ 1,327]

I usually right click on the part of the site and then click Inspect Element. Then just look at the CSS and HTML, you can normally find it by looking at the CSS.