There is extensive documentation of what you need to do to release your app in the Android documentation. It is so much documentation that I wanted to gather a more straight forward list of steps.
Create a self-signed key
To generate your key, use this command:
keytool -genkey -v -keystore my-release-key.keystore -alias alias_name -keyalg RSA -keysize 2048 -validity 10000
You will be prompted for a password (You might want to use a strong one) for your keystore. Then you will be asked for some information and finally another password for your key (You can use the same as the one you used for the keystore). That will generate a file called my-release-key.keystore. Keep this file in a safe place because you will need it every time you update your app.