Placing Google Ads in an app

Hi, I've created my first app and got it published in the app store. Biiig progress for me.

So, it's been there a year now (I've been working on other projects) and only sold 5 copies. It's an app that 1,000s of people could use so I'm thinking of putting in the app store for free download but put some ads in it to generate revenue that way.

Question is, how do I put Google ads in to my app. Are there any resources or guides I could use to do that. 

I'm just near completing my second app which I also think will attract a big audience. Same question applies or more generally, what's the best way to make an app generate some income. I genuinely believe enough people would find both apps useful/educational/fun. So I know there's a market there.

Appreciate any advice from those further down the road than myself.

Thanks