First, check for local code requirements; it they are OK with it. You will need:
2 2 X 4 X 16', one to fasten to the trailer and the other for the end of the joists plus a 1 X 6 X 16' facia board.
9 2 X 4 X 10' rafters, one for each 24" plus one to begin with.
6 1 X 4 X 16' nailers to go across the rafters to screw the roof into.
4 4 X 4 X 8' posts or decorative posts.
A nice, light, bright covering would be fiberglass panels.
10 lbs. 16d coated nails, 5 lbs. gasket headed roofing screws, 5 lbs. 8d coated nails for the strips.
A handrail may be required depending on the height of the floor from the ground.