Normally your entries are individually wrapped in their own div tags. Most likely the class will be “post” (code located in your main template):
<div class="post">
Add a little php code:
<div class="post<?php if(!is_single() && $post==$posts[0]) echo '-firstpost';?>">
This code checks two things. 1) The current page must be a listing of entries, not a single entry’s permalink page, and 2) it singles out the first post in the loop array, $posts[0].
If both conditions are met, “-firstpost” is appended. So your first post’s div will be of the class “post-firstpost” and all other entries simply “post”.
From here on out, just edit your stylesheet.Create a new entry called “.post-firstpost” and add your style elements to customize.
Even easier:
Edit stylesheet. Duplicate the class “.post”, rename the copy to “.post-firstpost”. Now both are identical, a better starting point. Make changes as necessary.
Hope this helps you out! Make sure to search the forums before asking questions, as this may have been addressed before.