Forum: Ruby on Rails gem install rails - FAILS

Very new to Ruby/Rails, followed the download/install instructions, 
until
this step:

> gem install rails

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

        /usr/bin/ruby1.9.1 extconf.rb
/usr/local/lib/site_ruby/1.9.1/rubygems/custom_require.rb:36:in 
`require':
cannot load such file -- mkmf (LoadError)
        from
/usr/local/lib/site_ruby/1.9.1/rubygems/custom_require.rb:36:in 
`require'
        from extconf.rb:1:in `<main>'


Gem files will remain installed in 
/usr/lib/ruby/gems/1.9.1/gems/json-1.7.6
for inspection.
Results logged to
/usr/lib/ruby/gems/1.9.1/gems/json-1.7.6/ext/json/ext/generator/gem_make.out

On Wed, Jan 2, 2013 at 4:06 PM, Doug Royer <douglasroyer@gmail.com> 
wrote:
> /usr/local/lib/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require':
> cannot load such file -- mkmf (LoadError)
>         from /usr/local/lib/site_ruby/1.9.1/rubygems/custom_require.rb:36:in
> `require'
>         from extconf.rb:1:in `<main>'
>
>
> Gem files will remain installed in /usr/lib/ruby/gems/1.9.1/gems/json-1.7.6
> for inspection.
> Results logged to
> /usr/lib/ruby/gems/1.9.1/gems/json-1.7.6/ext/json/ext/generator/gem_make.out

Based on your paths I'm going to assume you are on debian-ruby so
install ruby1.9.1-dev then try again.