Even a 'flat' roof would require minimum slope, consider talking to local roofing contractor and see what options are available since we can't see any photos of your house.

Hi winda17,

If you're looking for visuals of how a finished room over a garage could look, there are many great examples here on Houzz. (Use those key words in the photos search box)

From a structural point of view, the existing garage walls and foundation/footings will need to be able to accommodate the new load and your garage ceiling joists will usually have to be either replaced or added to in order to handle the new live loads. (They were never designed for that) The slight slope you have on the current roof is usually created with tapered 2x material, which would usually be removed along with the roof surface you've described.

If you're interested in pursuing this project like this I'd suggest arranging for a site visit by an Architect, Structural engineer or qualified GC. Either one can all give you a good idea of what you'd have to do in order to build the addition.

Steve