Add a username to your database.yml, might as well use your application's name (or some variant of the name) as the username, I'll use app_name as a placeholder:

development:
  adapter: postgresql
  encoding: utf8
  database: app_development
  pool: 5
  username: app_name
  password:
Then create the user (AKA "role") inside PostgreSQL using psql.exe:

$ psql -d postgres
postgres=# create role app_name login createdb;
postgres=# \q
The first line is in your terminal, the next two are inside psql. Then do your rake db:create.

The User user is possibly a default but user is already taken for other purposes in PostgreSQL so you'd have to quote it to preserve the case if you wanted to use User as a username:

postgres=# create role "User" login createdb;
You're better off creating one user per-application anyway.

You'll want to do similar things for your test entry in database.yml as well