I second Pengwuino's suggestion. If you want to learn enough LaTeX so you can post a few expressions here, you don't need to know all that much. By looking at what others did, you can usually figure out how it works.
If you ever get to the point where you want to learn LaTeX to write papers, it's worth getting a book, like Kopka and Daly's Guide to LaTeX, that explains LaTeX concepts. You can find a lot of the same information on the web in bits and pieces, but it's nice to have it all in one place.