Install Locomotive: that will install Ruby, Rails, the required gems and MySQL for you, so all you need to do is configure Tracks. Jacken has a very clear tutorial on how to install Tracks using Locomotive. Ensure you read the comments for troubleshooting information.
Apache is already installed, so there’s very little to do to activate it. Simply fire up System Preferences > Sharing and click the checkbox beside ‘Personal web sharing’. This will start up Apache for you, and you can stop it again by pressing the Stop button on that page.
Marc Liyanage has a very easy to install package for MySQL and very clear instructions to get it up and running. I would also recommend using CocoaMySQL to administer your databases. It provides a friendly GUI face for the database.
Rather than duplicating excellent work, I’ll just point you in the direction of Hivelogic which has an excellent [url=http://www.webhostingart.com/]web hosting[/url] guide for building Ruby 1.8.6, Rails, FCGI and so on. There are also instructions there for configuring Apache to use FCGI and to use Virtual Hosts.
If you have Tiger, you’ll find that Ruby 1.8.2 and SQLite3 is already installed. However, there’s a small problem with installing gems, which you can fix by following the instructions here. I really recommend using SQLite3 as the database on Tiger. It’s much simpler than having to install MySQL, and perfectly adequate for a personal tool.