The Google Drive folder ID is in the URL when browsing the Google Drive website (drive.google.com).

It's the string after the /folders/ part of the URL:

In the above screenshot, the folder ID is: 1lter6bJp4xSw91litFMvKoEvz2qBgq5i

In EZ Exporter, you can also just copy/paste the entire URL and the app will automatically pull out the folder ID.

If you want to upload the file to the main/root folder ("My Drive"), simply enter: root