In your application there will be a main activity which starts at first when your application launch. This is your launcher activity. So you can store your password to preference and every time you open your app check the password that the user filled in the given EditText box against preference value. If it is okay pass the general flow of your activity to activity moving otherwise halt your app to open further or close the app. This check can be done with simple if-else statement in the first activity.
Hope your prob solved now