How do you upload stuff you made on GIMP?

This depends on what you want to upload your works for. If you only want to show your friends a picture you made, the related links contain image hosters that allow you to upload images for free. After uploading, you receive a link to your image online that you can give to your friends.

Other internet portals allow you to share your works with large communities. You will most likely have to create an account there, which is usually free.

Another possibility is of course getting access to an online file server and uploading your files there via FTP conection.