Once you have your APK file, you can publish your app to any Android App store, including Google Play Store.
To publish your app on Google Play Store, you will need to buy a Google Developer Account. This currently costs $25USD, paid to Google.
Follow the steps :)
First of all, open this LINK to go to your Google Developer Console.
#1: Click on "Publish an android app in Google Play"
#2: Complete the form
#3: Click on "Upload your first apk..."
#4: Upload your apk file
#5: Go to advanced mode
#6: Click on "upload new apk..."
#7: Click on "Save Draft"
#8: Go to "Store listing"
#9: Fill in your title and your short and full descriptions
#10: Add 2 screenshots and an Icon :)
#11: Complete the form and tick the checkbox
#12: Save draft and go to "Content Rating"
#13: Click on "Continue"
#14: Add your email twice
#15: Click on "Reference, news..."
#16: Tick NO in every checkbox
#17: Click on "Save questionnaire"
#18: Click on "Continue rating"
#19: Click on "Apply rating" and go to "Pricing and distribution"
#20: Click on "Select all countries"
#21: Click on "No ads"
#22: Tick the checkboxes
#23: Click on "Save Draft"
#24: PUBLISH :) :) :)
Now you will have to wait till Google Play accept your app. This process may take one or two days.
Enjoy using your App!