Connect light fixture to a junction box
It looks like you plan on using the pull chain.  If that’s the case I think you need a black wire from the fixture to the black marette and a white wire from the fixture to the white marette. If there is a terminal on the fixture for ground you would join it to the other grounds as well which I think have their own marette if I’m looking at that picture correctly. There are several experienced people involved with this conversation who you should consult with first, but I believe it’s the easy answer.