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.