Welcome to learn the basics for android.

If you are new to android get the latest version of the ADT bundle to start developing apps for Android
use the below link
http://developer.android.com/sdk/index.html

download the latest zip file specific to your operating system(windows, linux, mac). Un zip the file and launch the eclipse and your IDE for android development is ready.

If you already have an IDE check out the below link for the development setup
http://developer.android.com/sdk/installing/index.html

watch my tutorial to setup the IDE for windows
http://youtu.be/QzOe4vBQOw0