You would normally use an ad service such as AdMob, AirPush (Android Only) or TapJoy amongst many others. You might also consider AdWhirl, which utilises mulitple networks.
Don't expect much income from ads unless your app is reasonably popular, but you can find more specific numbers by looking at the websites of various services -- they all provide this sort of information up-front. How you choose to integrate the ads and the types of ads shown also play a huge part in the potential to earn an income.
Hope that's helpful!
I've looked into air push and tap joy, they dont really state clear of the rates. Even they do say something like how much $ click per thousand impressions, it doesn't give me an idea of actual numbers, even approximate would be great for making my financial plan.
About expectation, indeed I don't expect too much (do I? : D). Perhaps I should explain: I'm looking at 50,000~80,000 DAU (if facebook), and I'm figuring how long it takes to generate $5,000 from ads.