Help! I'm SO confused about creating my simple one-page website!?

I already have a hosting account, which has an existing domain that someone built for me. My hosting account allows me to add five more domains, and I have instructed my hosting account to add another domain, and they did. But unfortunately, their sitebuilder will only work with my original domain, not any added domains. Therefore, I'm being told that I have to create my new domain's pages, music links, etc. separately, and then upload them to my existing account under the new domain. I've been shown where to upload files under my new domain, but when I do and then log on my my new website, I just see the file name. If I click the file name, it will come up, but that's not how a website should work. Additionally, I want to add a link on my site to play a song. What am I up against? Do I sound like a total novice or what? Please help. Thanks!