Community
You can get started here:
http://usa.autodesk.com/adsk/servlet/index?siteID=123112&id=18162650
Also, if you have questions on .NET API, you'd better post to .NET forum, instead of ObjectARX forum.
Norman Yuan
If you are looking .NET applied to AutoCAD, then check http://www.autodesk.com/myfirstautocadplugin
For generic .NET material, there are tons on the web, each one focused on specific topics. I personaly like books, like "C# Step-By-Step", but that a personal choice