You can't just tap into this circuit without knowing what it provides power to first. It could be switched power or it could be power to a kitchen or bathroom and in that case you are not allowed to tap it. And if it is a circuit that you can tap you won't be able to do it through the little hole in the ceiling.
You put the cart before the horse cause you should of thought of how you were gonna bring power to this fan BEFORE making your hole. If you don't have access above this ceiling then you're gonna have to bring power up there via a different method, like from a wall receptacle in this room. To do this you'll have to make a hole in the joint where the wall/ceiling meets and drill a hole through the top plate to run a cable from your receptacle. You'll also need to install a switchbox to control this fan unless you buy a remote controlled fan. This link will help in showing you how to run the circuit:
http://www.taunton.com/finehomebuild...ges/h00070.asp
You'll need to install a fan brace between the ceiling joist like what's on this link:
http://www.westinghouseceilingfans.com/ac_fanbrace.html
This will allow you to install it through the hole you've made. And, if you live in the Chicagoland area where only conduit is allowed then you'll need to install individual wires in conduit. Make sure the wire gauge matches the existing circuits wire gauge that you tap into.
You can also run wiremold up the wall, across the ceiling and to the fan and your cable is installed inside the wiremold. This is sold at Home Depot and can be painted to match the walls.