First you need to have the APK file of your app
Once you have created a Google Play Developer account (and pay the $25 fee to Google), go to https://play.google.com/apps/publish and log in to your Google Play account.
1. Click All Applications and then click Create application.
2. Select your Default language, enter your app Title and click Create.
3. The Store Listing tab.
4. In Product Details enter the Short and Full description of your app.
5. In Graphic Assets (Phone), click BROWSE FILES or drop a screenshot to add screenshots of your app.
You must provide at least 2 screenshots of your app.
6. Click + Add high-res icon or drop the icon to add your app icon (512×512).
7. Click + Add feature graphic or Drop a Feature Graphic. This image will be displayed at the top of your Store Listing page in the Play Store.
8. You can also add a Promo Graphic and a Promo Video (YouTube video URL) to promote your app, but it is optional.
9. In Categorization, select your Application Type, Category, and Content Rating.
10. In Contact Details, enter your Website, Email and Phone number (optional).
12. Click Save Draft.
13. Click on the App releases tab, and click on Manage Production.
15. For the message Google Play App Signing, click on OPT-OUT.