cron job not working - what am i doing wrong?

[unwashed potato!]

i've never used cron before so unsure what i'm doing wrong, but unfortunately the script i've got from online doesn't work.

below is the script i have which is save as 'mysql_autobackup.sh'

#!/bin/sh
#change directory to your backup directory
cd /home/websiteNameFolder/db_backup/;
#get backup of database of applications
mysqldump --user='[removed username]' --password='[removed password]' mydb >databaseName.sql;
#compress it in zip file
zip app_database-$(date +%Y-%m-%d-%H:%M).sql.zip tmp_db.sql;
#remove  sql file
rm -rf tmp_db.sql;
#delete backups older than 20 days
find /home/websiteNameFolder/db_backup/app* -mtime +30 -type f -delete;

i've edited out some bits.

then on cronjobs i have the following to run sunday at 4am using the following.

0    4    *    *    0

/home/websiteNameFolder/db_backup/mysql_autobackup.sh

but nothing happens. i checked the folder and no backup there. is there a way to see why it failed?

one thing i'm not sure on if where it says "mydb >" i assumed this was my database name so put that there, but then it has the .sql so wasn't convinced i was correct.

thanks