If a folder does not exist, create it

As others have said, use System.IO.Directory.CreateDirectory.

But, you don't need to check if it exists first. From the documentation:

Any and all directories specified in path are created, unless they already exist or unless some part of path is invalid. If the directory already exists, this method does not create a new directory, but it returns a DirectoryInfo object for the existing directory.

Use the below code as per How can I create a folder dynamically using the File upload server control?:

string subPath ="ImagesPath"; // Your code goes here

bool exists = System.IO.Directory.Exists(Server.MapPath(subPath));

if(!exists)
    System.IO.Directory.CreateDirectory(Server.MapPath(subPath));

Just write this line:

System.IO.Directory.CreateDirectory("my folder");

• If the folder does not exist yet, it will be created.
• If the folder exists already, the line will be ignored.

Reference: Article about Directory.CreateDirectory at MSDN

Of course, you can also write using System.IO; at the top of the source file and then just write Directory.CreateDirectory("my folder"); every time you want to create a folder.