Getting started with RubyOnRails For Macintosh 

STEP 1: Go to brew.sh the official website for downloading home-brew. What home-brew does is it adds files missed by apple while setting up mac.

In the website you will find a link saying copy and paste the link in terminal- do that and hit enter.

when you hit enter terminal starts to download the missing files and set it up for you.

it will say hit return key to continue or control + c to abort- hit enter(return)

then terminal asks for password simply enter the password with which you log in to your mac - while entering password dots will not appear in terminal as it usually appears while entering password so make sure that you enter the password right because entering garbage value in terminal might lead you to loss of data or so forth.

the procedure goes on . after completion you will see your mac name and dollar sign then simply enter :
  brew install rbenv (in the terminal of course )

this will take you to further installation procedure where terminal will download data from internet- it will take some time depending on your internet connection speed.

after completion following message will appear (this message is for my mac named : Notorious MAC)

To enable shims and autocompletion, run this and follow the instructions:

  rbenv init

==> Summary

🍺  /usr/local/Cellar/rbenv/1.0.0: 36 files, 61.9K

Macs-MacBook-Pro:~ Notorious$ 

then enter : brew install ruby in the terminal.

this will initiate downloading and installing process ..

now enter: ruby -v ( this will show the current version of ruby that you just installed in your mac.)

to install rails enter : gem install rails ( this will install rails and take some time.)

That's it Now you are good to go with Ruby on Rails .