As others said, you need to learn ASP.NET and learn to code. Start learning here: .asp.net/">ASP.NET | The ASP.NET Site[^
]. And perhaps this will give you some guidance too: Learning ASP.NET: Where to Begin?
Alternatively, you could find some open source forum projects built on top of ASP.NET and start from there, or you could hire someone that would do it for you which going to cost you alot of this.