Can't debug my asp.net web app developped on visual studio 2015 and hosted on IIS

I am a telecom engineer, I have studied a little bit of coding but I am not a professional, I have to develop a c# asp.net app for my company and I have some issues with debugging my app with Visual Studio 2015. I hosted my app on IIS (not IIS Express, even if I don't understand the difference), when I hit debug (with Firefox, Google Chrome...) a message of error displays where it is written:

unable to start debugging on the web server the remote server returned an error 400

I have litterally tried everything, can you please help me and give me solutions that I will understand (I am no professional), if you need a screen shot or something please let me know.