You could try this: Lay out a wooden timber for the "foundation" that goes from the metal wall out at 90 degrees, across and back to the building in "u" shape the same size as the greenhouse you want. Of course you'd remove all the things you have stored in that space first to make the work go smoother.
Then fasten an upright board to the building on each side as tall as you want the greenhouse roof to be. Place another board across these two at the top. Fasten all joints with metal fasteners.
So now you have a "u" shape on the ground and one on the side of the building. Place two upright pieces at the outside corners (away from the building)and brace them. They need to be as tall as the front wall will be, less a few inches and shorter than the back wall, to allow rain to run off. Put a couple or 3 more upright studs spaced along the wall. Place the rafters at each end and as many between as you need to hold up whatever roofing material you will use, but no more than 2&1/2 feet apart or so. Add a door on one end and any more vertical studs to hold your covering.