Create App Store Listings
In order to submit your app to the app stores, you'll need to create your app store listings. This is also where you would change any information related to the app during an update, so it's important to be familiar with the process.
Create iOS App Store Listing
First, make sure your developer account is paid for and approved, and any agreements are accepted.
Login to https://appstoreconnect.apple.com/
Click the plus button and add a new app.
Fill out all the information, including screenshots and pricing. Save as a draft.
Contact us to submit your .ipa file build, once that is done you will go back to the app store listing. Visit the Build section, click the plus button, add the build, save and submit for review.
Make sure you have a paid developer account.
Login to https://play.google.com/apps/publish/
Click Create App at the top right, fill out the fields and save.
Fill out all the fields in the "Main Store Listing" menu item.
Some areas require the .apk to be uploaded first. To do that, go to your build and preview tab, and select Submit to Google Play, and build. You will be emailed a link to download your .apk, you can then upload that to Google Play.
Create App Screenshots for Listings
You will need to generate screenshots for your app store listings. They must be a certain size.
See this article for help making great looking app store screenshots.