Forgive me if there's some standard way, but I don't use Red Hat much. For what I do know, you can put your custom startup scripts in your /etc/rc.d/rc.local file. So in that file, if you put a line that says
./etc/init.d/httpd start
It should start on boot
HTH
[edit]Ok, just found it. At the shell prompt, type "setup" to get into the RH Setup utility, from there go to System Services, and you can choose httpd from the list. Either way here will work[/edit]