We recommend using Ruby Version Manager it will ensure you're always on the currently recommended Ruby version and cleanly separate your Diaspora installation from all other Ruby applications on your machine. If you opt for not using it ensure your Ruby version is at least 1.9.2-p320, prior versions are incompatible. Be aware that this version is not maintained with security updates from the Ruby core team anymore. We currently recommend using the latest release of the 2.0 series.
As the user you want to run Diaspora under, that is not as root run:
curl -L dspr.tk/1t | bash
and follow the instructions.
Set up RVM
Ensure the following line is in your ~/.bashrc:
[[ -s "$HOME/.rvm/scripts/rvm" ]] && source "$HOME/.rvm/scripts/rvm"
Now close all open terminals and open a new one!
If you don't have sudo installed or your current user doesn't have the privileges to execute it, run:
rvm autolibs read-only
Closely watch the output of the next step for something like "Missing required packages:". If it appears, hit Ctrl+C and install all the packages listed there. Then rerun that command.
Ensure the currently recommend version of Ruby is installed:
rvm install 2.0