Make an entity (object) a class. Convert all business function of an object into methods.
If the basic behavior and function of some different entities are same then declare a base class for them and inherit it into each class. If some of the entities have same function but the way of functioning is different for each then declare an Interface for them.

Try to follow message passing architecture using methods like Transaction.MakePayment(int AccountNo, float Salary) etc.

Try to follow n- tier Architecture. Here is the list of some common layers.

 Presentation GUI
 Presentation Logic
 Business Logic
 Data Access
 Physical Data

Thanks,
Gurjinder Singh Brar
http://agileguru.blogspot.com