Let's say that you want to add some introduction text to the homepage and lets say we want it to appear above the Hot Picks and below the Search Box.
Open the index.tpl file (NOT index.php!). The easiest way to insert new content is to think of the page central content as a number of horizontal areas of content so you need to insert a new div, give it a style class (use an existing one or create a new one) and then insert your new content. For example:
From:
...
<div id="sw_container">
<div id="sw_content">
{include file="elements`$DIRECTORY_SEPARATOR`search_box.tpl"}
</div>
<div id="sw_content">
{include file="elements`$DIRECTORY_SEPARATOR`hot_picks.tpl"}
</div>
</div>
...
To:
...
<div id="sw_container">
<div id="sw_content">
{include file="elements`$DIRECTORY_SEPARATOR`search_box.tpl"}
</div>
<div id="sw_my_new_style">
Write whatever you want and lay it out however you want, add some more div's, etc...Welocme!
</div>
<div id="sw_content">
{include file="elements`$DIRECTORY_SEPARATOR`hot_picks.tpl"}
</div>
</div>
...
Then create a new style (easy to copy an existing one and adjust) in your css and style your content. A good style guide - http://www.w3schools.com/css/ - lots more out there!
You can repeat this process anywhere - it is a good idea to view the source of a page and use the style classes to find which tpl file you should add your new content to. The php files control the 'flow' of the output, tpl files contain the 'layout'.
I hope this helps.
Paul