If you are not competent with Visual Studio, DotNet and either VB.NET or C# you should start there.
A little familiarity with HTML and Javascript can't hurt as well, but is not required.
Then you should dive into Silverlight. You can start by picking apart ESRI's samples on the Resources site. To make the site your own, you should get a book on Silverlight or take an online tutorial. If you are into the graphics end and making it flashy, look into Expression Blend.
When you feel "advanced" enough, then look into MVVM or other modeling techniques.
Good Luck