This is a video tutorial on how to get started using C# and Webforms, should help you.
Can you post your code though so that I might be able to show you what you are doing wrong?
Here is the sample code I have for asp.net
Then you want to make a Handler (.ashx):
public class Upload : IHttpHandler, IRequiresSessionState
public void ProcessRequest(HttpContext context)
HttpPostedFile file= context.Request.Files["Filedata"];
int id = (Int32.Parse(context.Request["id"]));
string foo = context.Request["foo"];
file.SaveAs("C:\\" + id.ToString() + foo + file.FileName);
Post your code and I will take a look at it. Sounds like you are pointing to a resource that doesn't exist. Maybe your 'uploader' property is not pointed to the proper resource or your jquery link is broken (or not there).