Hmmm.....floor doesn't look like dirt, unless there's a lot of something laying over the dirt, looks like maybe tiles?
Is that a pile of rocks to the left?
What is that long angled board on the back wall?
Is it attached or just leaning there, wondering if it nailed on there to keep the building from racking (tilting to the side)?
First thing to do would be clear out that space, does not look like a small task.
Then take a good look at what's there. Look closely for any evidence of water infiltration, either from under the walls(run off from roof and surrounding area) and from above due to any roof damage.
Adding a floor may be a good idea...or not.
Could make a better surface for you and your birds,
but could also provide a haven for rodents to live under.
Starting with an existing building can be a huge advantage,but still lots of work.
Best part about it is having storage space out of the weather and separated from the birds. Check out My Coop link under my avatar for some ideas.
Lots of things to think about.
Knowing your climate would help.
Please put your location in your profile?