I think you need a little more detail, but I'm gonna take a shot at your questions. If you're not supporting any real weight from above ie. roof load then your single top plate is probably ok. you refer to roof, and I'm thinking you mean ceiling inside your shed, since this is under the patio cover it's already got a roof, correct? Same thing on your windows if you're not supporting any significan roof load then I'd make a header out of 2-2x4's on edge over the top of your window opening and butted right up to your single top plate. for the door opening, if you have the room I'd build a header out of 2x6 on edge nailed togather just because of the weight of the doors, don't want any sagging. If nobody else comes in here and answers this better then I am, pm me and I'll try and explain in a little more detail.
Chuck