You will need to build a number of trusses based on the length of your roof.
How to support shed roof.
Usually trusses are put 24 inches apart.
Essentially you will be building a series of triangular shaped trusses based on the pitch of your roof.
You will also want to consider the roof slope so that the building fits in with other buildings in the neighborhood and works well with the selected roof covering.
Bracing for putting up trusses.
In addition you may want to incorporate a shed loftwithin the roof space or a shed skylightto maximise the light in the shed.
To hold them up until the decking of the roof is placed on top a 12 foot 1 inch by 4 inch piece of lumber is used.
Another thing you can do inside your garden shed is installing additional support beams underneath the roof.