I chose phpbb3 for my forum. I want my user's post to be voted/rated by other members of my forum and also a user's reputation(ranking) should be changed according to his interaction with posts. Are there any built in features for these two tasks? If not what is the possibility to change the code according to my requirements? Any help is appreciated.
There aren't any built in features for reputation and ranking.
However, phpbb has a large database of user-built modifications which will definitely have what you need. See the mod database: http://www.phpbb.com/customise/db/modifications-1/?from=submenu&sid=b3bee6baf8411639512659054d6521c6
As for coding it yourself, you're certainly welcome to as far as licensing goes; but there will definitely be an existing modification which should suit your needs.