you want to add its init script to the approriate run level. The init script is typically /etc/init.d/apache2 where you could manually run /etc/init.d/apache2 start to start it.
on Gentoo you would write:
rc-update add apache2 default
On Ubuntu/Debian this works:
sudo update-rc.d apache2 defaults
on Redhat/Fedora a little googling shows this:
chkconfig --add httpd
It varies a little bit from distro to distro, but the idea is usually the same. Basically what all these commands do is make a symbolic link from /etc/init.d/ to the appropriate run-level folder in /etc/.