Hello, welcome to BYC.
First, look around this site--there are tons of pictures and descriptions to give you ideas.
Second, to get things cheap use Craigslist and/or go to building supplies recycling centers like the ones run by Habitat for Humanity. Also check out retailers or lumber yards giving away free pallets--they can be taken apart or used as-is to build a coop and run.
Third, consider all these factors as you finish building the coop. 1. Ventilation--lots of air flow needed over the chicken's heads to remove ammonia from the air they breathe, but you don't want drafts directly on the chickens. I would raise the height of that coop significantly so you can enter the coop yourself without bending over, and while you're doing that put in open spaces all under the eaves of the roof for ventilation (make the gap from the top of the wall to the roof at least 3-4 inches high and extend it the full length of the two walls that will catch a cross wind the best). Also add some windows that can be opened on hot days as heat is harder on the chickens than the cold.
2. You don't need a peaked roof like you'd see on a house--you could build a roof that slants in only one direction if that'd be easier to build but you do need some slant to shed water and snow.
3. Did you mean 8 nesting boxes or 8 chickens? A 3x5 coop only has 15 square feet and if you want 4 sq ft per bird that limits you to about 4 birds. Three or four birds can easily share one nesting box (my five hens often all lay in the same nesting box because they lay at different times of the day). Maximize what space you do have by using hanging feeder/waterer (or putting them outside if this is an option) and mounting the nesting boxes to the outside of the coop so you get the eggs from outside--this'll be necessary anyway if you don't raise the roof.
4. If you want to raise chickens cheaply then consider using the deep litter method with your own yard clippings and fallen leaves as bedding (read up about this--you need info to do it right). Another way to lower the bottom line is to let them free range for some of their food if you have the land for this. If you want to use the deep litter method then I recommend 5 sq ft per bird while limits you to 3 birds in the coop. Get a high egg producer like the ISA Brown, Red Star, New Hampshire Red, Rhode Island Red or Leghorn to get 2-3 eggs per day from 3 birds.
5. Final advice: make sure the nesting boxes are lower than the roosts so the birds don't roost in the nesting boxes.
You can use the search feature on this site to look up specific topics you need info on. Time spent doing that will be well worth the effort before you do any more work on your coop and run. Good luck!
