[Rails] install Rails from terminal

I have been having trouble install Rails from the terminal on my new
MacBook. I have tried to find a solution on both google and youtube, but
I have not find a solution to the problem

$ rails -v
Rails is not currently installed on this system. To get the latest
version, simply type:

$ sudo gem install rails

You can then rerun your "rails" command.

$ sudo gem install rails
Building native extensions. This could take a while...
ERROR: Error installing rails:
ERROR: Failed to build gem native extension.

/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby
extconf.rb
mkmf.rb can't find header files for ruby at
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/ruby.h

Gem files will remain installed in
/Library/Ruby/Gems/1.8/gems/json-1.6.6 for inspection.
Results logged to
/Library/Ruby/Gems/1.8/gems/json-1.6.6/ext/json/ext/generator/gem_make.out

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