If you have a large enough shed in your garden, it is possible to convert it into a perfect office! There are a few things you would need to do to make it the ideal office space. Obvioiusly, the main thing is to ensure you have a safe power supply in your shed, if you need to run computers and other electrical parts. This will need to be done by a qualified and regulated electrician for safety purposes.
Another aspect to consider is warmth in the shed. As these buildings are normally designed for storage and not for actually spending much time in, they are not insulated in any way. It is worth adding some form of insulation to the shed if you want to sit in it more comfortably.
Windows can be a good way to bring in additional light to your shed. If they do not currently exist on your shed, you could get them fitted.