How to create absolute url for a page/node? I think that I need to use the l() function as seen here: How do I get the absolute URL when using l()?

I just need a bit more clarification. What's the best way to create links on pages? I would like to link them correctly so that if my domain changes then the link will be updated (for example going from a local host to an online host). Let me also know if this is not the best way to do it.

I guess I need to know what would go into HTML tag  and then also do I need to place any code anywhere else?
You should use the l() and url() functions as specified in the answer to the question you link to. Those are core Drupal functions so nothing else is needed.