Please help me about, can we add Meta tag and description manually, without using any plugin in wordpress, if Yes!!! Then how, mean in which file,
You can edit "single.php" to look up the "meta" values you enter against a post but remember if you change theme you'll need to change the new files. The great thing about plugins is that the changes are independent of the theme you use.
You know how when you edit a post you can add custom fields, well give them a consistent name and then you can just ask for the custom field by name and echo it out as meta tags. You may, however, be better avoiding the coding and going for a plugin: http://www.google.co.nz/search?q=wordpress+meta+tag+plugin
check this http://codex.wordpress.org/Meta_Tags_in_WordPress Code (markup): header.php (theme file) file contains meta tags
I also use All in seo pack plugin, it works so well so far. Here is what i did to configure all in seo pack plugin and fill the meta tags and description of your blog. Any feedback will be appreciated.