What sort of server are you running out of? Whatever it is, there's probably a file with a name something like index.html somewhere near the top of the directory structure you're serving. Changing the content in that will change what you're showing to the world.
Past that, you're in the territory of tools and techniques. Google jQuery, or even just HTML, and start reading and experimenting - you can save HTML files on your local PC and look at them with your browser. If you want a nice free editor, grab yourself a copy of Java and a jEdit.
Getting stuff from your PC to the server can be another stumbling block, but the ways around that depend on the tools you're using.