Well, For Server-Side in Asp.Net commonly used languages are C#.Net and VB.Net. Prior knowledge of Classic ASP or PHP may be an advantage to learn web based technology like Asp.Net, but certainly it should not be hurdle if you do not have knowledge of it.
As you are already having knowledge of HTML, CSS and JavaScript which is definitely going to be useful for you in Asp.Net development.
You may choose either of below languages(C#/VB) for Server-side development of Asp.Net. I have used both in past and found C# more interesting(others may have different opinion :)).
For C# Programming you may refer below links.
Learn Visual C#
http://msdn.microsoft.com/en-us/library/618ayhy6%28v=VS.100%29.aspx
For VB Programming below links.
Learn Visual Basic
http://msdn.microsoft.com/en-us/vstudio/hh388568
For Asp.Net you may begin with below link.
Get Started with ASP.NET
Happy learning. :)