How to create a directory on server

You can get the path of server's root directory using

Request.PhysicalApplicationPath

Now assign this path to string variable

string pathToSave = Request.PhysicalApplicationPath;

Now create a directory on the server
Append the Folder name to the string variable

pathToSave  +=  "folderName" 

System.IO.Directory.CreateDirectory(pathToSave);

Now your directory is created.