Some questions

[dukemahajan]

How can i change the title of the forum home page?

Does the title of the page changes according to the subject of thread posted?  or it remains the same

Does the search engine indexes every thread in the forum?

can u please tell me the path of the file where we can change the meta tags?

[Oldiesmann]

For the forum title: Admin -> Edit Server Settings
Yes, the title does change according to the subject of the topic posted.
Some search engines do, others don't. It might take a while for them to get to every topic, because the time they spend on your site depends on how busy it is, among other things.
To change the meta tags, you'll need to edit Themes/{yourtheme}/index.template.php ({yourtheme} = the theme you're using).