To obtain a Maps API key you will need to specify the MD5 fingerprint of the certificate used to sign your application.  This means you will have to obtain the MD5 fingerprints of the debug and production certificates.

If you are you using Eclipse with the ADT plug-in, you can find the location of your keystore in the Default Debug Keystore textbox located in menu option Windows -> Preferences -> Android -> Build.  To generate the MD5 fingerprint for your debug certificate, use the Java keytool command, as shown here:

keytool -list -alias androiddebugkey -keystore <your_debug_keystore_location>.keystore -storepass android -keypass android

To generate a keystore for your production certificate, do the following:

1.  Create a folder to hold the keystore, for example: c:\android\release

2.  Use the Java keytool  to create a self-signed certificate, as shown here:

keytool -genkey -v -keystore <Path from step 1>\release.keystore -alias androidrelease -storepass <password> -keypass <password> -keyalg RSA -validity 14000

Now paste your certificate’s MD5 fingerprint in the appropriate field on this Google Android Maps API site.