Always try to match the house roof pitch angle and overhang for your shed roof. Some shed roofs are designed to be nearly flat. Flat roofs are more susceptible to leaking than pitched roofs. Shed roofs project as a single plane roof pitched away from the house. If you are tying the roof directly to a house wall you can install a ledger board with flashing and install rafter ties to attach the rafters at the appropriate angles or use engineered trusses. The opposite side of the porch roof will be supported by a header beam that is installed parallel to the house on top of 6x6 support posts. Support posts should be spaced less than 8' apart and be located on top of properly sized frost footings. Rafters should be attached to the header beam using hurricane ties.
If you are tying the shed roof into an existing roof on the house you will need to remove the shingles and sheathing to expose the roof framing. You will need to attach the porch rafters to the house roof rafters and replace the roofing and flashing.
Many building departments require a local architect or engineer to design plans for porch roofs because of the increased snow, roof, and wind loads created by adding a roof.