You want to add its init script to the appropriate 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 Red Hat Linux/Fedora/CentOS a little googling shows this:
chkconfig --add httpd
It varies a little bit from distribution to distribution , but the idea is usually the same. Basically, all these commands make a symbolic link from /etc/init.d/
to the appropriate run-level folder in /etc/
.