Brother... I've been there. The articles here are great, but they do assume (with all due respect) a little bit of UNIX geerkery. Check out their starting articles such as learning the Mac OS X terminal. * As for your 2 questions:
Load index.php by default:
1. Edit the httpd.conf file by typing: sudo pico /etc/httpd/httpd.conf (This line means that you want to edit (pico is an editor) the file 'httpd.conf' (located in /etc/httpd/) and you want to do it as a 'superuser' (hence the sudo command).
2. Enter your admin password. (this is necessary because of the 'sudo' command)
3. After the file is opened (in pico) hold down the Control key and type w. -- This command will allow you to enter some text to search for.
4. The cursor will be in a white bar at the bottom of the terminal, labeled 'Search:'. Type index.html, and hit the return key.
5. The cursor will move to a line that reads "DirectoryIndex index.html". Move your cursor to the end of this line and type 'index.php'. Make sure there is a space between 'index.html' and 'index.php'
6. Save the httpd.conf file by holding down the Control key and typing o. Hit the enter key to confirm the save.
7. Exit pico by holding down the Control key and typing x.
8. Restart Apache by typing: sudo apachectl graceful, enter your password if/when prompted.
The alias to the local host is a little bit more involved. If the behavior you are looking for is: Typing your domain name (www.mysite.com) into your web browser displays your local web server, then post your email address as a response and I'll send you a document that will help you out.
* I take no responsibility if you screw up your system using the steps I described above.