Follow the below steps to generate .apk file in android studio.
Open android studio . Build -> Generate Signed APK… . now click on “Generate Signed APK”.
Click Create new.
Fill the details and click ok.
It will go back to previous window.
Click Next, and give the password which you stored in key.
Now click Finish and wait to complete the building process.
Now apk generated successfully. Click how in Explorer.
Copy the apk file and install in android mobile.
If you already created the .jks key then skip the step 2. provide the master key password to use the existing key.