If you haven't already done so, install XCode (it's on the Tiger DVD), and fink. If you've only just installed fink, make sure you do a
Those will take a considerable amount of time if you haven't run them before.
Now you need to install MySQL. You can either do it from the bare tarball, or use a package manager like fink or darwinports. I like fink, so do:
fink install mysql mysql-client
and let fink grind away at all the assorted dependencies mysql needs. This will take a while, MySQL is big.
Now that MySQL is installed, we can install rails -
sudo gem install rails
Rails doesn't ship with an adapter for MySQL. We install it like so (this should be one line)
sudo gem install mysql -- --with-mysql-lib=/sw/lib/mysql --with-mysql-include=/sw/include
so that it can find the mysql libraries fink installed earlier. Note that the -- --with-mysql-include is supposed to be there, that isn't a typo.
You can now go find one of the Rails tutorials like the one Vincent Foley has at http://darkhost.mine.nu:81/~vince/rails/tutorial.html and get started with Rails.