Depends on what type of app it is.

>>1366934
Whatever you do, don't have ANNOYING ads, make them easy to ignore, but visible enough to imprint a memory in the user's head.

Take a look at Twitter for example. They don't to the whole "ad banners and blinking buttons" thing, they just do sponsored tweets in your feed. You have to be smart like that.

Also, if you absolutely must be an asshole with the advertisements, at least make the app buyable to remove ads. If I encounter too many ads, especially pop-ups, I WILL delete your app.

Find someone who can make you an app and pitch him your idea. Ask him how long and how much it'd cost. There's plenty of places that have freelance people to hire.