Step 1: Check Java Version

Open command window and type the following command

$ java -version.

if output is less tha 1.6.

Update the java version to java 6 and above

Step 2: Generate Keystore

use the command:

$ keytool -genkey -v -keystore PATH/TO/YOUR_RELEASE_KEY.keystore -alias YOUR_ALIAS_NAME -keyalg RSA -keysize 2048 -validity 10000.
Where:

"PATH/TO/YOUR_RELEASE_KEY.keystore": is the path in which you want to store the file and "YOUR_ALIAS_NAME": is the name of keystore remember.

Step 3: Attach Keystore to the Unsigned Android App

Type this command:
$ jarsigner -verbose -keystore PATH/TO/YOUR_RELEASE_KEY.keystore -storepass YOUR_STORE_PASS -keypass YOUR_KEY_PASS PATH/TO/YOUR_UNSIGNED_PROJECT.apk YOUR_ALIAS_NAME. Where PATH/TO/YOUR_RELEASE_KEY.keystore: The path to your keystore. YOUR_STORE_PASS: is the password you selected when you created a keystore.YOUR_KEY_PASS:This should be the same as YOUR_STORE_PASS unless you have changed it.PATH/TO/YOUR_UNSIGNED_PROJECT.apk:is the path to your unsigned app. YOUR_ALIAS_NAME: is the alias you selected when you created a keystone