Leesa-
Learning Compressor and all it's mysteries is a fine idea. However, when I am in a hurry, there is a GREAT shortcut for this in Quicktime.
Open your video file in Quicktime, then look in the file menu for "Export for Web". That feature will make you three different size movie, a poster frame, and give you sample code and instructions for putting it on your web page.
Now the fine and talented folks here are already reaching for there keyboards here to beat me up about being able to do a much better job with the toolset in Compressor. And they are right, they can. Bit this will get you on the web this afternoon.
-Vance