If you're just testing the waters, an inventory marketplace like BuySellAds.com might be worth checking out.

You can browse through the inventory of similar sites to get a gauge of what you can expect, CPM wise: https://buysellads.com/buy/allsites/

Later on, you might just want to strike one-off deals with interested advertisers, or build in a more "native" ad solution into your platform, etc.