如何在课堂上决定方法

I am new to Object Oriented Programming. I am developing a software for a Grocery Store. The Grocery Store has Customers, Customers has Address and Subscriptions. All these are different classes in the application. I am little confused, that in which class should I create which method and how I should decide this. Like viewSubscription() should be a part of Subscription class or Customer class. modifyCustomer() should be a part of Customer class or Store class. I am new to Object Oriented Programming. I am d