Hello Chathuri,
Do you want the course rating to be a 'fun only' thing, or do you need to capture and use the results?
If it's just for fun, then there is a block your can install (see http://docs.moodle.org/en/Course_rate_block). This just gives a star rating (out of 5 stars) but does not say how many people have voted.
If you want to capture and use the results more fully, then you'll probably need to use the questionnaire (http://docs.moodle.org/en/Adding/editing_a_questionnaire) as the results of this will show in Gradebook.