I think you should start using paragraph tags if you write in paragraphs on your site. Breaks are breaks, paragraphs are paragraphs. They have a different semantic.
I suggest starting out on a good freehost and then moving up to a good paid host. I suggest you first learn XML, then XHTML, CSS, and Javascript. Then you will be ready enough to make a perfect website.
I think you are having code issues. hr is a horizontal rule, not a span. Just change your HTML to <span style="position:relative;background-image:url(images/hr.gif);width:611px;height:1px;"><h3>Introduction</h3></span>
Umm... HTML does DO things. It defines Data. It can't add x+y but it can tell you that they are variables. It can't explain what you are defining but it can tell you that something is a definition. It isn't moving mountains but it is still doing something.
People are making this too complicated. You need to know HTML and CSS, and if you want to make things hard, Javascript. There is an article on A List Apart about them here: http://www.alistapart.com/articles/hybridJust edit the CSS a bit to make it vertical.