help me

C:\ruby-2.0.0-p247>cd DevKit

C:\ruby-2.0.0-p247\DevKit>ls
bin         devkitvars.bat  dk.rb  include  m.ico  msys.bat  postinstall  share
config.yml  devkitvars.ps1  etc    lib      mingw  msys.ico  sbin

C:\ruby-2.0.0-p247\DevKit>ruby dk.rb init
[INFO] found RubyInstaller v2.0.0 at C:/Ruby200-x64

Initialization complete! Please review and modify the auto-generated
'config.yml' file to ensure it contains the root directories to all
of the installed Rubies you want enhanced by the DevKit.

C:\ruby-2.0.0-p247\DevKit>ruby dk.rb install
[INFO] Skipping existing gem override for 'C:/Ruby200-x64'
[WARN] Skipping existing DevKit helper library for 'C:/Ruby200-x64'

C:\ruby-2.0.0-p247\DevKit>gem install rails
Temporarily enhancing PATH to include DevKit...
Building native extensions.  This could take a while...
ERROR:  Error installing rails:
        ERROR: Failed to build gem native extension.

    C:/Ruby200-x64/bin/ruby.exe extconf.rb
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
        --with-opt-dir
        --without-opt-dir
        --with-opt-include
        --without-opt-include=${opt-dir}/include
        --with-opt-lib
        --without-opt-lib=${opt-dir}/lib
        --with-make-prog
        --without-make-prog
        --srcdir=.
        --curdir
        --ruby=C:/Ruby200-x64/bin/ruby
        --with-atomic_reference-dir
        --without-atomic_reference-dir
        --with-atomic_reference-include
        --without-atomic_reference-include=${atomic_reference-dir}/include
        --with-atomic_reference-lib
        --without-atomic_reference-lib=${atomic_reference-dir}/
C:/Ruby200-x64/lib/ruby/2.0.0/mkmf.rb:430:in `try_do': The compiler failed to ge
nerate an executable file. (RuntimeError)
You have to install development tools first.
        from C:/Ruby200-x64/lib/ruby/2.0.0/mkmf.rb:515:in `try_link0'
        from C:/Ruby200-x64/lib/ruby/2.0.0/mkmf.rb:813:in `try_run'
        from extconf.rb:24:in `<main>'


Gem files will remain installed in C:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/atomi
c-1.1.12 for inspection.
Results logged to C:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/atomic-1.1.12/ext/gem_
make.out