The which command can be used to determine if the createdb command is in your $PATH.

~]$ which createdb
/usr/bin/createdb

If using the default posgres user account, I typically go with the following command. In this example, a database named "mydb" is created.

sudo -u postgres createdb mydb

If you have created a user account, the -u or --username option and --password flag can be used.

createdb --username johndoe --password mydb

Better yet, you can setup passwordless authentication using the hidden .pgpass file and then connect without the --password flag. In this scenario, you would need to be in johndoe home directory since the hidden .pgpass file should be in johndoe home directory.

~] cd /home/johndoe
~] psql --username johndoe --dbname mydb createdb mydb

Or, the --command option can be used.

~] cd /home/johndoe
~] psql --username johndoe --dbname mydb --command "create database mydb"

Or you could set the PGPASSWORD environment variable.

~] export PGPASSWORD='itsasecret'
~] psql --username johndoe --dbname mydb --command "create database mydb"
~] unset PGPASSWORD