Ok, here is the first step, download the Android SDK, its freely available.http://code.google.com/android/download.html
Also it is advisable to also download the IDE (Integrated Development Environment) called Eclipse, while it is not the only "option" out there some of the truly old school types can roll-there-own using emacs or vi as a text editor and maybe configuring by hand the Java path.
Here is the download link for Eclipse
http://www.eclipse.org/downloads/
There is a plug-in that is required to develop Android under Eclipse
http://code.google.com/android/intro/installing.html
I myself can verify it installs and works correctly on MacBook (Intel) for both the Eclipse IDE, the Android plug-in and Android SDK.
On Windows it should install easily, but I do not have a Windows machine and cannot verify this.
On Linux, I am not sure, but if it installs on the Mac it should install on Linux platforms as well.



