Using RVM and installing rspec gem for Ruby

I think I installed rvm correctly. When I type in rvm -v it outputs rvm 1.25.33 (stable) by Wayne E. Seguin , Michal Papis [https://rvm.io/] However, I am trying to install the gem rspec. I type in gem install rspec but the output is ERROR: While executing gem ... (Gem::FilePermissionError) You don't have write permissions for the /Library/Ruby/Gems/2.0.0 directory. What should I do? What is going wrong? I can't understand the rvm docs either!