I just installed wordpress and i want to know someone please suggest me which contact us plugins is better and easily installed my site
Most plugins can be installed from the dashboard. I haven't used Wordpress in a while but I think you can filter the search results by rating and/or most downloaded. This would give you a good indication of which is best.
Contact Form 7 is the name of the easiest plugin, but search a little bit, maybe you find something better.
You need to put Contact form on a page, not a post. Or you go to Appearence > Editor > Page Template and remove the call to comments.php
install FIREBUG mozila addon and check the 'class' for recent comments . Then, change the color of that class in style.css file in your theme folder
Contact 7 plugin and jetpack also has contact form if you use that.. i think he means you manually edit the comment box by using firebug to spot the code..