Install Ruby and other needed libraries

sudo apt-get install ruby irb ri rdoc ruby1.8-dev libzlib-ruby libyaml-ruby libreadline-ruby libncurses-ruby libcurses-ruby libruby libruby-extras libfcgi-ruby1.8 build-essential libopenssl-ruby libdbm-ruby libdbi-ruby libdbd-sqlite3-ruby sqlite3 libsqlite3-dev libsqlite3-ruby libxml-ruby libxml2-dev

Install Ruby Gems

wget http://rubyforge.org/frs/download.php/35283/rubygems-1.1.1.tgz
tar -xvvf rubygems-1.1.1.tgz
cd rubygems-1.1.1/
sudo ruby setup.rb
sudo ln -s /usr/bin/gem1.8 /usr/local/bin/gem

Install Sqlite (probably already installed)

sudo apt-get install sqlite3 libsqlite3-dev

Install Imagemagick

sudo apt-get install imagemagick

Install Rails

sudo gem install rails

Install other Gems

sudo gem install sqlite3-ruby mongrel capistrano mongrel_cluster mini_magick

If you need MySQL

sudo apt-get install mysql-server mysql-client libdbd-mysql-ruby libmysqlclient-dev