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I have found that using a computer can add more frustration to a small project than help. I admit it… learning a new program can be fun! At the same time, it is not always necessary to reinvent the wheel!
There are many books that have plans available. Pick one close to the dream tool shed you want and modify it, rather than starting from scratch. Use your local library to search books and back issues of home improvement magazines. Some mags have annual indexes. Your library may even be able to search their archives by topic to help you locate the right articles.
One really good reason to use a pre-made plan is the designers have already done the hard work… designing a structure that won't blow over with the first good gust of wind! A CAD program will not do this for you.
Depending on the size of the structure you envision, you may need to contact your local building inspector Be sure your structure conforms to local codes... they are designed to establish a consistent level of structural safety for all buildings in your community. OK… we are not into promoting bureaucracy here, but most building codes are not tools of social engineering, just an aid to our survival and safety!
Hope this is helpful! Good luck… and email or fax us a photo of your shed when you're done. Love to see it!!