I connect to database quite rarely (over the console). So often I forget simple things. This is my start for PostgreSQL database, you just installed it (9.4 version).

I will use console commands.

Create user and database:

$ sudo -u postgres createuser -DRSP myuser 
Enter password for new role: 
Enter it again:
$ sudo -u postgres createdb -O myuser mydb

-D, --no-createdb - The new user will not be allowed to create databases. This is the default.
-R, --no-createrole - The new user will not be allowed to create new roles. This is the default.
-S, --no-superuser - The new user will not be a superuser. This is the default.
-P, --pwprompt - If given, createuser will issue a prompt for the password of the new user. This is not necessary if you do not plan on using password authentication.

UPD: Remove user

$ sudo -u postgres dropuser -i myuser 
Role "myuser" will be permanently removed.
Are you sure? (y/n)

Thats it.