BLT79

I have installed ceiling fans before with no problems until now. I just moved into this old duplex where the ac is crap and it's hot as balls outside. I finally break down and get a ceiling fan to install. So I get up to the light fixture,pop it off and to my surprise there aren't enough wires. There are four wires coming from two lines,both sets are black and white. The black wire from one line is connected to the light and the white wire from the other line is also connected to the light. And then the other black and white wires from the seperate lines are connected together. And oh I almost forgot there is no ground wire in sight. What do I do here???