How do I set up dev environment with buildkit?
As Kurund pointed out, the general instructions are at https://github.com/civicrm/civicrm-buildkit . Follow those instructions to download the all the tools.

There are a few details that may not be well represented yet in the documentation:


As the main developer behind buildkit, I use it mostly in MAMP (and a bit in Debian and Ubuntu).
Buildkit runs several things on the command-line, so you need to make sure that the command-line is setup with the right version of php. By default, OSX uses its built-in version of php, but MAMP's version is generally better. See: http://wiki.civicrm.org/confluence/display/CRMDOC/Setup+Command-Line+PHP
I suggest putting buildkit in your home directory (e.g., git clone https://github.com/civicrm/civicrm-buildkit.git ~/buildkit). This is where all the tools will go. The source-code for CMS+Civi may go somewhere else (more in a moment).
After downloading buildkit, you'll need to setup a site with Civi. You can choose among two distinct workflows:


gitify: You install CMS+Civi manually and then run gitify to load the bleeding-edge Civi code.


Where: You can put the code anywhere. In MAMP, /Applications/MAMP/htdocs is probably easiest.
Pro: You can use any tools or processes you want to setup Apache, MySQL, CMS, etc.
Pro: Your configuration is custom. You control everything directly.
Con: Your configuration is custom. It's hard for other developers to reproduce, and it doesn't match the official test/demo environments.
Con: If you need to create multiple sites, you have to do all setup work repeatedly.
Con: If you want to work with the automated tests or schema, then you'll need to do several extra configuration steps.
Summary: gitify is generally best for light development with a single build.
Next step: Setup Apache+MySQL as you normally would. Install a CMS (Drupal/WordPress) and CiviCRM using tarballs or zip files. Then, run gitify with no arguments. It will display a help screen with some examples.

civibuild: You run a command like civibuild create d46 --url http://d46.localhost; the command automatically configures Apache+MySQL, downloads Drupal, downloads Civi v4.6, and installs them.


Where: All builds will get placed under  buildkit's folder. For example, the "d46" build might be ~/buildkit/build/d46.
Pro: Setting up new sites is automated. It's easy to create several different sites using a mix of CMS's and versions.
Pro: Your configuration is standardized. It matches the official test/demo environments.
Pro: Each site is automatically configured to run the automated tests.
Con: Your configuration is standardized. Loading things to match your personal preferences requires more work.
Con: The upfront configuration can be difficult/unforgiving - buildkit needs access to the internals of Apache and MySQL. Generally, it's based on a philosophy of "fail early, fail hard".
Summary: civibuild is generally best for heavy development.
Next step: See the detailed tutorials at https://github.com/civicrm/civicrm-buildkit/blob/master/doc/civibuild.md or https://buildkit-ui.civicrm.org/#/welcome .


If you go the civibuild route, then will need to prep the system by running amp config, which will present some questions and instructions for the local system. In a default MAMP configuration, some good values are:


db_type: mysql_dsn
mysql_dsn: mysql://root:root@127.0.0.1:8889 (or sometimes :3306)
perm_type: osxAcl (or none)
perm_user: www
hosts_type: file
httpd_type: apache
httpd_restart_command: /Applications/MAMP/Library/bin/apachectl graceful

In any workflow, buildkit has better support for Drupal- or WordPress-based dev-sites. There's some experimental code to support Joomla-based dev-sites, but the Civi-Joomla installer is difficult to automate, and it probably won't be resolved until http://forum.civicrm.org/index.php/topic,35050.0.html gets some real attention.