Hi Scaredy, and welcome to the forum.
You've come to the right place if you want to make your own coop on a budget, and a garden shed can be successfully converted at relatively small cost. If you browse the Search box, top right, in the General Chickens section, you will find several long threads where people have shared their experiences and asked wuestions about design and conversion methods, so then do feel free to give us some more ideas about what you hope to plan, and I expect you will get lots of help.
Planning the run is equally important, and you need to decide the maximum number of birds you have room to keep, based on the calculation of an absolute minimum of 2 square metres of run space per bird (more is better, don't overcrowd them or there may be social problems such as bullying and feather pecking. The run has to be strong and foxproof, with sides made from weldmesh, not chicken wire, which will keep the birds in but wouldn't exclude predators. Again, you will find help on previous threads, plus answers to the questions you'll have as the project develops.
You may not have noticed the main Poutrykeeper site, see link at the bottom of the page, which has some useful sections on starting up with keeping chickens, housing, feeding etc, as well as lots of other advice.
http://poultrykeeper.com/