Install Xcode, Ruby, Rails, Git, Homebrew, Mongo on New Mac OS X

Xcode

Download Xcode (this can take a while). After installing go to xcode preferences via the menu bar click on downloads -> Install (next to Command Line Tools).
Check if gcc was installed correctly.
gcc -version

RVM (Ruby Environment Manager)

Download and install rvm using Curl

bash <<(curl -s https://rvm.beginrescueend.com/install/rvm)

Once this runs without any error add the below code into your bash profile. If it does not exist create one ~/.bash_profile.

[[ -s "$HOME/.rvm/scripts/rvm" ]] && . "$HOME/.rvm/scripts/rvm"

After this you can check for different Ruby implementations

rvm list known

rvm list (this will display the Ruby versions installed on your machine).

Ruby

This one is pretty simple now just add the version of ruby at the end of rvm install.

rvm install 1.9.3

To use a particular version of Ruby

rvm use 1.9.3

To make a particular version of Ruby default

rvm --default use 1.9.3

To use system default version of Ruby

rvm use system

Homebrew

Homebrew is a package environment for OS X. It installs the missing packages

ruby -e "$(curl -fsSL https://raw.github.com/mxcl/homebrew/go)"

$brew should not throw any error

Rails

gem install rails

Git

Install git using brew

brew install git

Configure git with your username and email

git config --global user.name "name"
git config --global user.email "email"

Mongo

Install mongo using brew

brew install mongodb

start up using mongod