So the big 8x12 shed is in my back yard.
I'll assume that first photo is your shed?
tarp walls with hay insulation.
Why? What are your reasons to do this?
Where are you, by the way. Location can be a very important part of these decisions and suggestions. Climate, for sure but even things like are you north or south of the equator.
The left door will have the automatic door on it and the right door will be people door. I want to minimize the area and ducks getting pooped on… 15 chickens and 2 ducks
All that being said WWYD roost ideas! I put a couple of photos of what I found on the internet.
Is 15 chickens what you will have in the future or will that number grow? Plan for what you will have, not just what you have now.
Assuming the shelves are what I'm seeing in the back of that first photo, I'd probably widen the top shelf so it is 3' wide. That becomes your droppings board. Above that put two 8' roosts across the back. The first 12" off the back wall, the other 12" past that. My personal preference is for two roosts at the same level but you can vary the height if you want. The chickens won't mind.
It looks like that window opens up from the bottom. Put hardware cloth on the outside of the window so you can open it in warm weather for ventilation and still keep predators out. This is one reason why having a clue on your climate helps. How cold a winter and hot a summer do you expect? I also have an 8x12 coop with one window. I open it in the spring and close it in the fall.
You will need permanent ventilation winter and summer in addition to what you get with that window in warmer weather. That looks like a flat roof, not sure how it's finished. I don't know how I'd add ventilation without knowing what you are working with and your climate, but my first thought would be to rip out an area up high on both side walls somehow.
I'm not sure what your goals are for having chickens or your management plans. I would not turn that area in the back under the roosts and droppings board into the nests, that 3' board sticking out would probably get in your way of gathering eggs. I'd consider turning that area underneath into a brooder, a broody buster, a place to isolate a chicken that needs isolating, or maybe storage.
I'd put the nests on one wall where it is easy to get to them to collect eggs. Four nests is plenty for 15 hens. You can either have a horizontal row of 4 at a level lower than the roosts or a stack of two on two as long as they stay lower than the roosts. The ducks probably will use nests on the ground. Some of the chickens might use ground nests too. You never know what any one hen will prefer.
Of course all this is based on my goals, preferences, climate, and management techniques. I play with genetics, hatch chicks with an incubator or broody hens, keep some flock replacements, and eat the rest. The poop on the droppings board goes onto the compost pile. They forage for some of their food. I have no idea how this relates to you. My main guiding motivation is to make it as simple and convenient as I can for me, the chickens can adapt. I consider room important as the tighter I pack them the more behavioral problems I have to deal with, the harder I have to work, and the less flexibility I have to deal with things that pop up.
If you do use those shelves in the back for nests, I'd put two 8' roosts along one wall. Doesn't matter if they are the same height or there is some variation in height but I'd want them together as that leaves more wall space if you need it to deal with something.
My guess is that those shelves are 8' in length and 2' in height. It's hard to get a depth, maybe 12" deep? You have all kind sf options as to how you lay out the nests and what you do with the rest of that shelving. I don't have a preference.
if this happens I’ll put up blockers. 
