Architecture :: Modelling Application Layers?

I'm trying to create my first Asp.Net application from scratch. Tried to create this post in some of the other forums but I couldn't find a better place.

So, as the title says, I'm having trouble trying to model my application. I'm like an OOP newbie. I'll show my classes first and then I'll write my questions.

Model Layer

public class User
{
private int id;
public int Id { get { return id; } set { id = value; } }
private string login;
public string Login { get { return login; } set { login = value; } }
private string password;
public string Password { get { return password; } set { password = value; } }
}
Data Access Layer
public class BasicDAL
{
private SqlConnection connection;
//>>Returns a new connection to the database
public SqlConnection GetConnection()
{
//>>If connection is active close it before starting a new one
if (connection != null)
if (connecion.State != ConnectionState.Closed)
connection.Close();
connection = new SqlConnection("MyConnectionStringHere");
return connection;
}
}
public class UserDAL : BasicDAL
{
//>>Insert a new user
public void Insert(User user); { /* Insert into DB */ }
//>>Authenticate user login with its password
public void Authenticate(User user); { /* Authentication code here */ }
}
Business Logic Layer

[Code]....

So, model layer is pretty simple. Just a user representation. On the Data Access Layer I have a basic class with a GetConnection method. All DAL classes will extend this one.

My first problem lies on Business Logic Layer. With the above scenario, I've placed the same methods that I placed at the DAL. BLL methods would call DAL. As simple as that but I believe It is not the best way to do it, is it? How can I improve those classes?

Also, I have to "try..catch" blocks. That's because I can't find good places for it. I mean, if I place a try..catch on GetConnection method, for instance, how my ASPX page would get this error? How my ASPX page can tell the difference between "database offline" and "sql syntax error" when executing "userBll.Insert(newUser);".

My problem is mainly placing the exception handlers. I understand I would probably have to change return type of some methods. I didn't change because I believe that will have something with the exception handlers.

Btw, please assume I can't use TableAdapters and stuff like that. I would like to create all layers by myself.