I have to make changes in header. Just keywords and titles. Every page should have different keyword. but the problem is if i change header it applies on all pages. what to do now?
Well, if you love programming in PHP then it should be an easy fix. 
I have file called header.inc.php
This is just a small portion of my meta tags for SEO (I know I could have done this better, but I was lazy when I wrote this).
[php]if (preg_match("/HTML5, CSS3, jQuery and PHP Forums/i", $pageTitle)) {
$metaDesc = ‘’;
}[/php]
and this is in he header as well:
[php] <?php echo $metaDesc ?>
As you can see, this is how you can have multiple keywords / titles in the header for many different pages.