Error in set up Rails

Error in set up Rails References I am trying to learn Ruby on Rails. I have installed Ubuntu in virtualbox under the host system Windows 7 64 bit. I have set up Ruby and next step I want to set up Rails. After entering the command: sudo gem install rails I got the following messages: Fetching: minitest-4.7.5.gem (100%) Fetching: atomic-1.1.13.gem (100%) 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/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in require\': cannot load such file -- mkmf (LoadError) from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:inrequire\' from extconf.rb:13:in `\' Gem files will remain installed in /var/lib/gems/1.9.1/gems/atomic-1.1.13 for inspection. Results logged to /var/lib/gems/1.9.1/gems/atomic-1.1.13/ext/gem_make.out What should I do to solve this situation? I am ready to give any additional info you need. Thank you for your help.