how to create a folder in android External Storage Directory?

Do it like this :

String folder_main = "NewFolder";

File f = new File(Environment.getExternalStorageDirectory(), folder_main);
if (!f.exists()) {
    f.mkdirs();
}

If you wanna create another folder into that :

File f1 = new File(Environment.getExternalStorageDirectory() + "/" + folder_main, "product1");
if (!f1.exists()) {
    f1.mkdirs();
}

The difference between mkdir and mkdirs is that mkdir does not create nonexistent parent directory, while mkdirs does, so if Shidhin does not exist, mkdir will fail. Also, mkdir and mkdirs returns true only if the directory was created. If the directory already exists they return false

The use of Environment.getExternalStorageDirectory() now is deprecated since API level 29, the option is using:

Context.getExternalFilesDir().

Example:

void createExternalStoragePrivateFile() {
    // Create a path where we will place our private file on external
    // storage.
    File file = new File(getExternalFilesDir(null), "DemoFile.jpg");

    try {
        // Very simple code to copy a picture from the application's
        // resource into the external file.  Note that this code does
        // no error checking, and assumes the picture is small (does not
        // try to copy it in chunks).  Note that if external storage is
        // not currently mounted this will silently fail.
        InputStream is = getResources().openRawResource(R.drawable.balloons);
        OutputStream os = new FileOutputStream(file);
        byte[] data = new byte[is.available()];
        is.read(data);
        os.write(data);
        is.close();
        os.close();
    } catch (IOException e) {
        // Unable to create file, likely because external storage is
        // not currently mounted.
        Log.w("ExternalStorage", "Error writing " + file, e);
    }
}

void deleteExternalStoragePrivateFile() {
    // Get path for the file on external storage.  If external
    // storage is not currently mounted this will fail.
    File file = new File(getExternalFilesDir(null), "DemoFile.jpg");
    file.delete();
}

boolean hasExternalStoragePrivateFile() {
    // Get path for the file on external storage.  If external
    // storage is not currently mounted this will fail.
    File file = new File(getExternalFilesDir(null), "DemoFile.jpg");
    return file.exists();
}