Hi ..To install ruby do the following..
There are 3 ways to install
1)Open the terminal and type
sudo apt-get install ruby
It will ask for the password after giving password it will install ruby.
To check ruby is installed in your system type the following command
It will return the version of the ruby installed in your system such as
ruby 1.8.7 (2010-01-10 patchlevel 249) [i486-linux]
2)To install ruby from ubuntu software center
Open the ubuntu software center and search ruby then
click the ruby to Install.
3)To install by synaptic packet manager
Open the synaptic packet manager
Search the ruby package
Mark Ruby for installation
Click Apply in above panel
It will install ruby in your system..
Now you have successfully installed ruby in your system and now start working on it.