You can generate the Android .p12 certificate using Java Keytool utility that comes with any standard JDK distribution and can be located at %JAVA_HOME%\bin. On Windows this would usually be C:\Program Files\Java\jre7\bin.
So on Windows, open a command prompt and switch to that directory and run below command after changing the "AppName" with your application name.
You will be asked to set a password, first name, last name, Org, city, state country etc...
The .p12 file will be generated and saved to same folder in which you are in. Here validity 10000 means 10000 days.