Apache is installed and enabled in OSX by default. To confirm and check your version instance, see this.
To start it, run this command in terminal:
sudo apachectl start
Then open up http://localhost on your browser and you should see this:
You can also check the threads in Activity Monitor.
To find out where this page is being served from, find the file httpd.conf. This file resides in /etc/apache2/
Then search for “DocumentRoot”. It should say /Library/WebServer/Documents at line 246. This is where the index.html file is being served from.