Developing a Data Access Layer for C# applications

Hello all,
I've taken up a project on developing DAL for c# applications. my task is to write libraries containing wrapper classes that allow straight-forward execution of SQL Queries, Stored Procedures and Scripts against a database. And provide methods to support all functions related to Database access. I've to write Database Access Layers for SQL Server, Oracle, MS Access, My SQL and PostgreSQL.
I'm sure there are many experienced C#.net programmers in this forum. I wud like to know how to go about this.... Thanks in advance.....