Note: it is Phonegap Build's policy not to retrieve signing keys for users, for legal reasons. Hold on to them.
Set Java_Home directory (http://docs.oracle.com/cd/E19182-01/820-7851/inst_cli_jdk_javahome_t/index.html).
Open the command prompt (cmd.exe) as an Administrator, then Run the following command:
$ keytool -genkey -v -keystore [keystore_name].keystore -alias [alias_name] -keyalg RSA -keysize 2048 -validity 10000
Keytool will ask for keystore password. Enter password and confirm:
Next, keytool will ask for additional information. Supply appropriately:
Next, keytool will ask password for Alias. Return if it's the same as keystore password. Othewise enter password and confirm:
Your signing key is now ready to submit:
Go to your Account > Edit Setting > Signing Key's tab.
Click 'add a key...', ensuring you use the same alias used when you generated your key.
Go to your Account > Edit Setting > Signing Key's tab:
Click unlock button and supply the the certificate password (from step #6 above) and the keystore password (from step #4 above)
Lastly, either set your key to be default using the checkbox in the keys list, or in your individual application's details, select the key you've uploaded and unlocked.