Currently the backwards compatibility is only for users in the Xbox Preview Program, but an update is expected to release soon to add the feature for all users.
Those in the program can play 23 games, of them includes games like the first Mass Effect, Banjo Kazooie, and Perfect Dark.
The good news is that Microsoft opened a support page to allow everyone to vote on what games they want to see. This is open to everyone, all you have to do is sign into a Microsoft account and vote.
So far it seems Call of Duty is dominating that list with almost every iteration since Modern Warfare at the top, and Black Ops 2 leading the way. There are some other top titles like the leading Red Dead Redemption or Halo and even Skyrim.
According to Microsoft once the update is released on Xbox One all that needs to be done by publishers is an approval process. There is no work on their side of things at all. So perhaps this list will tell Microsoft on who to approach first, and first parties are almost certain to be added.