You shouldn't need to mess with functions.php because this all sounds like a CSS issue as far as giving the home page a different design.
If you want a different styling of your home page, you can just wrap the entire page and use CSS to style things differently. Navigate to your Header and find
<body>. Change that to:
echo "<body id=\"home\">";
Now you can use CSS to style all elements on the home page differently than all elements elsewhere since everything on the home page can be called on by using
As far as displaying previews of 5 posts, it's probably
query_posts that you want to use:
Honestly, though, most themes are set up to display excerpts from the latest 5 posts.
A link to your site would help out.