Hi:

I'm creating one classified site, in that we need to write SEO friendly urls for google page ranking and optimisation. How to do that any one pls help me.

There are many tutorials that you can find within Google's web index (search engine, with a little more fancy terminology added).
Here's an example:
zenverse.net tutorial
Example of .htaccess code:

Options +FollowSymLinks

RewriteEngine On
RewriteRule ^topic/([a-zA-Z0-9]+)/$ index.php?topic=$1

This would route yourdomain.com/topic/{id} to yourdomain.com/index.php?topic={id}.

If you wanted lets say, profiles... you could do something like this:

Options +FollowSymLinks

RewriteEngine On
RewriteRule ^profile/(.+)/$ profile.php?user_id=$1

This example (I modified the one above) would route yourdomain.com/profile/{user_id} to yourdomain.com/profile.php?user_id={user_id}.

I hope this helps, and I recommend doing research before asking... this gets asked way too much!

Have a good day.