This is mostly geared towards using Ruby on Rails on the Mac OS X operating system, but it applies to any web language, PHP, perl, etc. Maybe not the best for Java but that's an arcane language for web development.
First thing, is to download and install MacPorts. MacPorts downloads and resolves dependenices for you. MacPorts
Run the self update sudo port selfupdate
To install git: sudo port install git-core. If you get a failure, ie. "Error: The following dependencies failed to build: rsync", be sure to do the selfupdate command.
Type in 'git init' in the folder you want to keep under revision control.
Install the git bundle for textmate.
You now have source control management for your project! Follow Raliscast 96 for a brief tutorial on git with Ruby on Rails.
More to come later...