Hi,
Gang of Four[
^] is very famous to learn about Design pattern. Initially you will find it difficult to implement in your project but when you have big project with complexity, design pattern make it easier to maintain.
It is not compulsary to use any of the design pattern but if you are developing long run project then it would be helpful to your team members to understand.
If you are beginner with asp.net then i suggest you to just learn basic concept of the C# and ASP.Net. After experiece you will find some difficulties in your application and at that moment this design pattern will be understand by you.
Example:
If you need one class that maintain some value, but that class is called many times but you don't want to create multiple instance of that then you can follow
SingleTon Design pattern[
^]
Hope you learn other pattern gradually,