One of the first things you need to do is to project the roof line out from the house.
If the house is 3:12 and the porch follows the same 3:12 roof line, how low will the eave be? Of course that depends on how low the house eave line is now.
It really depends on the size of the roof load and the spacing of the porch roof support posts. Most porch roofs usually have posts, say every eight feet or so. These could easily hold up a large timber and the rafters could set on it. With short braces from each post to the beam and with the proper size rafters your porch should look right and be a nice addition to your home.
Just some ideas to think about.
Whatever you do, have fun doing it!