I needed to have my minecraft server auto start when the server was powered on so after a couple of hours and lots or trial and error I found this helpful site and 5 minutes later my minecraft server was auto starting below is the final code used in my upstart script.
description "Start minecraft server upstart job"
start on (local-filesystems and net-device-up and runlevel [2345])
stop on runlevel [016]
chdir /home/$USERNAME/minecraft
script
su $USERNAME -c "screen -d -m -S minecraft_server java -Xmx1024M -Xms1024M -jar craftbukkit.jar"
end script