To initialize  get started with  Android, you'll need to  get familiar
with Git and Repo.

http://source.android.com/download/using-repo

To initialize your local repository use a command like this:

- cyanogen eclair tree (CyanogenMod)
  repo init -u git://github.com/cyanogen/android.git -b eclair

- snowfix eclair tree:
  repo init -u git://github.com/snowfix/android.git -b eclair

- main android eclair tree:
  repo init -u git://android.git.kernel.org/platform/manifest.git -b eclair

Then to sync up:

repo sync

After checking out the repositories, you'll need to create a
buildspec.mk file with contents like this (for dream/sapphire):

TARGET_PRODUCT:=cyanogen_dream_us
TARGET_BUILD_VARIANT:=eng
TARGET_BUILD_TYPE:=release

If you are building for the HTC Passion/Nexus One:

TARGET_PRODUCT:=cyanogen_passion
TARGET_BUILD_VARIANT:=eng
TARGET_BUILD_TYPE:=release

For additional instructions on getting the necessary proprietary
files into your build, and to get things going from here:

http://source.android.com/documentation/building-for-dream