A good shed is a safe, quiet and comfortable place for your guinea pigs to live in.
Safety and Security
There should be secure locks on the doors and windows and maybe even a shed alarm installed (available from DIY stores). A "beware of the dog" sign on the main door, even if you don't actually have a dog, is also a useful intruder deterrant. There should also be a fire extinguisher installed by the main entrance (available from DIY stores), and this should be serviced at regular intervals.
Comfort and Welbeing
Guinea pigs should have access to natural light and so your shed should have windows. If the windows can be opened this will also provide fresh air, although if your windows cannot be opened then DIY air vents (also available from DIY stores) installed into the doors will do the same job (a joiner should be able to do this for you). The shed should have methods for warming and cooling the temperature at various times of year. Electricity in the shed is not essential, but it allows the use of lighting, fans and heaters.
Storage
Your shed should have storage space for your hay, feed and bedding. Somewhere to keep other items such as towels, grooming equipment and spare bowls and bottles is also useful. I recommend keeping your medical box inside the house with you.
There's a lot of things you need to have in your shed before setting up your guinea pigs in their new home. I prefer to run a tight ship and have everything I need to hand so it's really easy to care for my guinea pigs.
Broom
This is essential to keep the floors clear of debris.
Dustpan and brush
Great for getting into difficult corners and removing debris from hutches.
Bin bags
A roll of bin bags kept in the shed is handy for quickly disposing of bedding and other rubbish.
Rubber gloves
Never be without a pair of rubber gloves when getting stuck into dirty bedding!
Disinfectant
Always keep a bottle of small animal disinfectant in the shed for cleaning out hutches and pens.
Newspaper
The essential bottom layer of every bedding.
Small fridge
If your shed has electricity it's definately a good idea to invest in a small second hand fridge to store vegetables and medications that keep well when cool.
Antiseptic wipes
Use to clean the surface you use to prepare vegetables.
Dustbin
Keep your dry feed in this to stop foraging and keep it fresh.
Fire extinguisher
With luck you will never have to use it but always keep a working fire extinguisher near the main entrance to your shed just in case.
Fly netting
Try and attach some fly netting to the windows of your shed to stop flies getting in. Flies are the cause of maggots and irritation to your pets so fly netting over open windows is a must, especially during the summer months.
DIY air vents
Some DIY stores sell cheap air vents which you can fix into wooden shed doors. These are very good if you can't open your windows because they still let the fresh air in. As with open windows, try to cover your air vents from the inside with fly netting.