How to recover or resent the password for the user 'postgres'

Hi,

I am trying to follow what has been mentioned below **Setting Up Postgres**
(https://www.digitalocean.com/community/tutorials/how-to-setup-ruby-on-rails-with-postgres).

But no luck!

[shreyas@myapp(add_postgres_addapter)]$ su - postgres
Password:
su: Sorry
[shreyas@myapp (add_postgres_addapter)]$

It is asking me the password, whereas I don't know the password of the user `postgres`. How would I create a new role
witha password in this case ? 

I am using OS X version 10.8.2
 Regards,
Arup Rakshit