Once a person has created an android application, he can offer his app free for advertising . There are many platforms which can be used by the app owner in this context. Some of them are AdMob, InMobi, Millennial Media, and TapJoy etc though there are many more like that. When the app user clicks on the advertisement the app owner would share the revenue with the advertiser. Let us see an example of an app which suppose worldwide 600000 people view in a month. It means ads are seen 600000 times in a month. This is known as impressions. Out of this how many times user has clicked matters most and suppose out of this 1000 users clicked the advertisement. So for every click you get say $0.03 (this rate would differ from ad to ad and platform to platform) and your revenue for the month becomes $30. One should remember that there are many apps in the internet but as the clicks on the advertisements are very occasional, it is not easy to earn a good amount from this arrangement. Only a few top apps might be earning a good amount.
For the gaming apps there is one another way to monetise and that is to have in-app purchases and premium upgrades mechanism inside the app and one has to monitor it and attend to the user needs time to time. This is a good model but requires a lot of coordination and administering work.
Another option is set the price of your app and upload it in Google Play store and offer it to user. If app is really useful, the users would purchase it and you get the money. You may have to upload a lite demo version of your app for the user so that he can test it before purchase. Once he pays for it he can download the paid version. There are many apps in the Google store that users purchase for their use. Some of the top paid apps today are Minecraft, TuneIn Radio Pro, Plex for Android, Monopoly, Plants vs. Zombies etc.
Knowledge is power.