There are two ways. You can make a static page- search this forum for lots of advice on how and why to do that.

The other way is to edit your skin so that every post has the same extra text on it, so long as you want it to be on every page, that is.

Add the following code to either posts.main.php or index.main.php in your skin directory

if( is_default_page() )
{
    echo 'Blog home page text';
}

alternatively you can add a Home Page container and use Free HTML widget to post the content in it

if( is_default_page() )
{
    skin_container( NT_('Home page'), array(
		'block_start'       => '<div class="homepage">',
		'block_end'         => '</div>',
	) );
}