Thumbnail post with post text preview help

start with creating a child theme of Twenty Twelve; http://codex.wordpress.org/Child_Themes

then create a copy of content.php from the parent theme into the child theme;

look into the content.php template, and you will see that the code for the featured image is already there, just in the wrond space, and that the theme alredy creates a excerpt, but only for search results.

there are already some forum topic dealing with adjusting and changing that.

when you get stuck with more detailed problems, please post a link to your site to illustrate what you have so far.

http://codex.wordpress.org/Post_Thumbnails