As far as design standards are concerined it is always a good idea to use the layered approach. Most designers will agree that there should be three layers. 1) Data Access Layer, 2) Business Logic Layer 3) Presentation Layer.
1) Data Access Layer: Tables needed to store your data and queries or stored procedures needed to add, change, update, delete, or query the data.
2) Business Logic Layer: Code used to execute the queries contained in the Data Access Layer as well as capturing the business rules that will cause these queries to be executed.
3) Presentation Layer: Forms and related controls that will display the information to the user.
The Presentation Layer calls the functions and procedures created in the Business Logic Layer and displays the results.
The Business Logic Layer provides the access to the database objects made available in the Data Access Layer.
Using this layered approach compartmentalizes development and makes it easier to locate and correct errors. It is however important that you finish one layer before starting work on the next.