Disposing and destroying instances

Hi all,

Do you know how to dispose instances?

examples:

clGeral Geral = new clGeral(); DataTable dt = new DataTable(); Geral.GetUsers(dt); Geral = Null;

clGeral is a class created by me, and there is no Dispose method inside because it is a simple class. It just opens SQL Connection, query and return some data, close SQL Connection.
Making Geral = null will solve my problem?  class will be destroyed or I need to do anything else?

Another exemple:

DataTable dt = new DataTable(); ... ... ... dt.Dispose(); dt = null;

In this case, DataTable have Dispose method, do I need to use dt = null after dt.Dispose ?  Or dt.Dispose is enought?

thanks!