I enabled ratings from the Document Library settings page and choose “Rating settings”. Once this setting is made a new column is added to my library .But after selecting a rating, it is not saving. please guide me how to save it and use it?
There is a timer job that helps aggregate the results. It is possible that the job isn't running. Here is a blog post I previously wrote about enabling those jobs.
The rating isn't saved directly to the db. You have to enable at least the services described in the article http://codename-srini.blogspot.com/2010/04/how-to-use-rating-control-in-sharepoint.html
It might be saving, but it might be taking a long time, since the timer job is not scheduled to run at shorter time intervals by default. Follow the post above from @MikeOryszak and you should be able to see it within the scheduled interval.