We are going to develop content Management System in ASP.net.Please suggest us what is the good design pattern do we need to follow in order to have good design.
What design are you looking for?
User interface? Even that question depends on who your user audience is going to be. You design a system for tech-savvy users differently than one designed so that "grandma" can use it.
Business Objects?
Database table and stored procedure design?
Coding standards?
This is way too vague for an answer that could be of any practical help.
I belief it's not the right start to search for usefull patterns first. Take a look at Graig Larmans Applying UML and Patterns: An Introduction to Object-Oriented Analysis and Design and Iterative Development
It's all about the design. First write down the requirements. What do you want to develop. What should the features be. The Applying UML book is very clear about this. I can't explain it in a few sentences.