Facing difficulty in Installation of Interactive Ruby shell and ruby in ubuntu 13.04 and getting started with rails

Facing difficulty in Installation of Interactive Ruby shell and ruby in
ubuntu 13.04 and getting started with rails

Accidently I have installed both versions of ruby on my system and now the
default version is set to $ ruby -v ruby 1.9.3p194 (2012-04-20 revision
35410) [x86_64-linux] but I need to set the default version to 2.0.0...
for this I used the command $ rvm 2.0.0p294 --default but it says rvm is
not currently installed though I installed it, typing various commands(via
google) . Also I want to install an Integrated Ruby shell in ubuntu 13.04,
do suggest how to do it. I have also installed Aptana Studio on my system,
now how should I get started with it. Finally, the last problem is tell me
how to check whether rails is currently installed in my system or not...
Thanx in advance!!