Answer: Very Easy!
Get the Code
svn co http://svn.codehaus.org/jruby/trunk/jruby
cd jruby
ant
Run Your Built JRuby
export PATH=$PATH:$PWD/bin
jruby my_script.rb
Install Gems
gem install somegemOR
jruby -S gem install somegem
Run the Benchmarks
jruby -J-server -O test/bench/bench_method_dispatch.rb(-J-server uses the "server" JVM and -O disables ObjectSpace)
Build and Test Your Changes
ant clean test
Look For Bugs To Fix or Report Your Own
JRuby JIRA (bug tracker)
Join the Mailing Lists
JRuby Mailing Lists
Join Us on IRC
Channel #jruby on FreeNode IRC
We're waiting to hear from you!