Add these lines to Gemfile

#PostgreSQL
gem 'pg'

Edit config/database.yml

development:
  adapter: postgresql
  database: mydb
  pool: 5
  timeout: 5000
  username: postgres
  password: ***********

Install gem, DB migration with rake

Yamada-Naoyuki-MacBook-Pro:hello yamadanaoyuki$ bundle install
Fetching gem metadata from https://rubygems.org/.........
Enter your password to install the bundled RubyGems to your system: 
Using rake (0.9.2.2) 
Using i18n (0.6.0) 
Using multi_json (1.3.6) 
Using activesupport (3.2.8) 
Using builder (3.0.0) 
Using activemodel (3.2.8) 
Using erubis (2.7.0) 
Using journey (1.0.4) 
Using rack (1.4.1) 
Using rack-cache (1.2) 
Using rack-test (0.6.1) 
Using hike (1.2.1) 
Using tilt (1.3.3) 
Using sprockets (2.1.3) 
Using actionpack (3.2.8) 
Using mime-types (1.19) 
Using polyglot (0.3.3) 
Using treetop (1.4.10) 
Using mail (2.4.4) 
Using actionmailer (3.2.8) 
Using arel (3.0.2) 
Using tzinfo (0.3.33) 
Using activerecord (3.2.8) 
Using activeresource (3.2.8) 
Using bundler (1.1.5) 
Using coffee-script-source (1.3.3) 
Using execjs (1.4.0) 
Using coffee-script (2.2.0) 
Using rack-ssl (1.3.2) 
Using json (1.7.5) 
Using rdoc (3.12) 
Using thor (0.16.0) 
Using railties (3.2.8) 
Using coffee-rails (3.2.2) 
Using jquery-rails (2.1.1) 
Using libv8 (3.3.10.4) 
Installing pg (0.14.0) with native extensions  // Here it comes!!
Using rails (3.2.8) 
Using sass (3.2.1) 
Using sass-rails (3.2.5) 
Using sqlite3 (1.3.6) 
Using therubyracer (0.10.2) 
Using uglifier (1.2.7) 
Your bundle is complete! Use `bundle show [gemname]` to see where a bundled gem is installed.

Rake

Yamada-Naoyuki-MacBook-Pro:hello yamadanaoyuki$ rake db:migrate
==  CreateBooks: migrating ====================================================
-- create_table(:books)
NOTICE:  CREATE TABLE will create implicit sequence "books_id_seq" for serial column "books.id"
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "books_pkey" for table "books"
   -> 0.0050s
==  CreateBooks: migrated (0.0051s) ===========================================

Listing tables on psql console

Yamada-Naoyuki-MacBook-Pro:hello yamadanaoyuki$ psql mydb
psql (9.1.4)
Type "help" for help.

mydb=# \d
                List of relations
 Schema |       Name        |   Type   |  Owner   
--------+-------------------+----------+----------
 public | books             | table    | postgres
 public | books_id_seq      | sequence | postgres
 public | schema_migrations | table    | postgres
(3 rows)