Windows XP (32-bit), Vista (32- or 64-bit), or Windows 7 (32- or 64-bit)
Eclipse 3.4 (Ganymede) or 3.5 (Galileo)
JDK 5 or JDK 6 (JRE alone is not sufficient)
http://developer.android.com/sdk/requirements.html

Develop to Android 1.6
http://developer.android.com/sdk/android-1.6.html

Supporting Multiple Screens
http://developer.android.com/guide/practices/screens_support.html

Quick Start
http://developer.android..com/sdk/index.html