In your case it's rather simple. First check which runlevel you're running at (runlevel is something like configuration). The command is
You'll get something like 'N 5'. The number is important. Now go to /etc/rc.d. Inside you should have directories like rc0.d, rc1.d and so on. There also should be one with the number you have. In my example above it'd be rc5.d.
Then create a symlink as root:
ln -s /etc/init.d/appname /etc/rc.d/rcX.d/S90appname
Replace appname with the right script name and X with the right runlevel number. Short explanation: rcX.d are directories with scripts started when changing runlevel (in your case it's important that at boot you enter the right runlevel). Scripts inside are run in order of the numbers (90 in this case means it runs close to the end of the boot process). S means to start a service.
It's done you can test it.