Once it's done, you can find your archive in the Xcode organizer. This is where all your previous versions and archive builds are saved and organized in case you do not delete them. You will immediately notice a large blue button saying "Upload to App Store..." however in 9/10 cases this will not work due to various reasons (Xcode bugs mostly). Workaround is to export your archive and upload it using another Xcode tool called Application Loader. However, since Application loader uploads IPA files to the App Store, archive needs to be exported to the correct format. This is a trivial task which might take ~ half an hour. Click on the "Export" button in the right side panel.
If you are uploading app to the App Store, select the first option and click Next. Sign in and validate your code once again and go grab a cup of coffee. Once the exporting process is done, you will be asked where to save the generated IPA file. Usually, desktop is the most convenient choice.