how can i highlight part of my post

If you use the HTML tab, you could place the highlighted content within something like <div class="highlight">[...]</div> and then add a background color via CSS:

.highlight {
background-color:#ffc;
}

is there a simple plugin for that?

maybe to add the code so i’ll only need to use shorcode in my posts next time?

Here is some code for you to add to your functions.php. You will need to tweak the style to suit yourself.

Be careful when changing functions.php – you can disable your whole site. Be sure to make a backup copy first, and understand how to copy it back before you make any changes.

<?php // [mm-hilite]some text here[/mm-hilite]
function mm_hilite_func($atts, $content=null) {
	extract(shortcode_atts(array(
		'noatts' => 'Unknown',
	), $atts));
	$output = '';
   if (!is_null($content)) {
		$output = "<span style='background: green; font-weight: bold;'>$content</span>";
   }
   return $output;
}
add_shortcode('mm-hilite', 'mm_hilite_func');
?>