You'll need to create a link from /etc/rcS.d to your startup script
$> cd /etc/rcS.d
$> ln -s /etc/init.d/express-server.sh SNNexpress-server.sh
Where NN is a number of your choice to determine when during the startup sequence you want your script to get called. If your script depends on networking, then it should be start after networking service was called.