How do I build a roof on a 20x12 porch?

An existing open porch?
A roof that will be held up by posts?
You need to put in piers and footings if they aren't there already, according to the code of your area (below the frost line). Where I live it's 42" deep. That's the main work.
Then there's the shape of the roof: shed, gabled, hipped, flat, almost flat. That might depend partly on the style of the house and what the wall is like that it will be coming off.
And so on...

Suuggest you go see how a house is built and get some books. You may need a permit . Could be relatively easy for a shed style and very challenging for a hip roof.

Do you plan on removing the 2x8's and the plywood? If not, they will rot and if so where is your support under this 3.5'' slab??

Roofs can be very complicated if you have not ever did one, I suggest you read some books or hire someone with experience