You have to register with an ad network that allows segmentation. I know that InMobi and Playnomics give you that option. Then you register your app and integrate the ad network SDK into your app. Once you've done so, you'll get the options to start campaigns(and a lot more!) in your dashaboard.
I work for InMobi so if you need any help at all with starting an ad campaign, do let me know :)