all my birds are on pony shavings
depending on if the bedding gets wet i clean out the bigger coops (8x12) every 3 months, but there are 5 bags of shavings inside, about a foot deep on the floor. fire some sunflower seeds in once a day and they'll keep the
turned in as long as it stays dry. if the waterer leaks or rain gets in i clean out the wet stuff right away. harder in the winter with the snow tracking in.
as for structure. is it metal clad? wood? vinyl? does it have any wood framing inside at all to screw things to?
roosts should be easily accessible height wise. some birds don't fly or jump as well as others. bigger meatier birds can get up further but can damage their legs jumping down (think sort of controlled fall to get off the roost). they need to be far enough away from the wall that a bird can perch without touching the wall with it's head or tail. if you put roosts near the roof give them the height of the bird away from the roof so they can stand and walk on the roost, and then some roosts closer to the floor to "hop up" to the top ones on. i found a 2x4 laid so the wide side is where their feet are is really good in the winter, because when they roost in the winter their tummies cover their entire feet so their toes don't get frost bit. you do not need to heat the coop in the winter. but a water heater for the waterer is incredibly handy.
nesting boxes.....take the number of birds you are going to put in there, and divide by 3 or 4. you will need that many nesting boxes. for standard sized chickens 12 inches wide by 10 inches deep by 10 inches tall is plenty big enough, unless you are getting jersey giants or something, then bigger. i have no clue for banties-don't have them. i just made mine out of plywood, using a 2x2 furring strip to screw the ply to, and then fit a plywood "anti-egg rolling out onto the floor" lip on the front. put a roost in front of the nesting boxes so they can jump up and get in. make the top of the nesting boxes on a angle so they don't sleep on it, and put the boxes up off the floor some so they don't crawl in to go to sleep. i put the roosts above the level of the nesting boxes so that the roost is a "better safer roost from predators" than the nesting boxes. last thing you need/want is
in the nesting boxes because they are sleeping in them!
hang your feeder off the floor using a chain and you'll discourage the mice from eating ALL your feed.
Ventilation: does the door have a window? use 1x1 hardware cloth, that will keep rats and weasels and coons out. you can staple it over the openings for any vents/windows etc. or i prefer using screws with large flat heads to catch the wire under, because i can take it out again without having staples everywhere. Plain old chicken wire will rust in about 2 years so use better quality hardware cloth. you will want to put screen in ALL the openings. including windows. animals like coons will climb to get in. you want the air flow to remove hot air in the summer and supply fresh cool air. in the winter you don't want wind blowing on the birds. the schematic ross has there is perfect. i would suggest a series of smaller openings, usually 12 inches by 4 inches (or that cool vent cover that ross posted-i'm cheap though-can't be bothered to spend the 3$ on a nice looking vent plate LOL) along the roof line, and then smaller ones nearer the floor, 3x3 or 4x2 or so to let the air in. a shed that size could do with just one vent really, it's not a huge area to exchange. again, you don't want gale force wind and snow coming in during the winter. you can make covers out of plywood to cover them but still allow air exchange by using eye hooks to hold the cover piece to the wall. again, make sure to seal with hardware cloth!
this should hopefully help. google searches also will help...someone here had some awesome nesting boxes built like a diamond shape...lots of great ideas findable!!!!
oh, and some people have great advice...build bigger than you think, and then build even bigger. they're right....having put in a 60x80 barn....LOL