The roof slope depends on a couple factors. Roofing type and local codes.

Shingled roofs need at least a 3/12 pitch. Most codes allow a 2&1/2 to 12 pitch, however. That is done so water does not wick up under shingles and cause damage and freeze in winter.

Roofs more flat that the 3/12 pitch(and the exception) will need rolled or membrane roofing.

To allow for air flow, you will need to make some kind of vents on each end parallel to the roof rafters. If the roof is a nominal peaked roof, a ridge vent at the peak and soffit vents at the ends will allow plenty of vemtilation.

This decision depends on many variables like how much room (height) there is available on the existing wall, the type of roofing you want to use, the climate, and how you intend to insulate it.

The wall will be between 8 ft and 7ft 6 in. I will use rolled roofing. I live very near the coast in San Diego.
I will use fiberglass insulation. Thank you for your help.