You must specify the database to use on connect; if you want to use psql for your script, you can use "\c name_database"

user_name=# CREATE DATABASE testdatabase; 
user_name=# \c testdatabase 

At this point you might see the following output

You are now connected to database "testdatabase" as user "user_name".
testdatabase=#

Notice how the prompt changes. Cheers, have just been hustling looking for this too, too little information on postgreSQL compared to MySQL and the rest in my view.