I'm using Ubuntu 10.04 LTS, and was trying to install the latest version of Ruby, 1.9.3. However, the command
sudo apt-get install ruby1.9.3
returns "Couldn't find package ruby1.9.3".
I'm not sure why that is, since it worked with ruby1.9.1.
The best (at least easier) way to install Ruby in Ubuntu is to install RVM (Ruby Version Manager).
There is a very good and simple guide written by Ryann Bigg available on his blog. Just follow each step and it should work!