So, you're tying the roof into an existing valley?
If so, you're in for some fun
The first thing to determine is which side of the deck you want the roof to drain into the gutter. Then you can start to determine how you want the roof to tie in with the existing. It may be helpful to tack a long board on the corner post and use a string from the roof to determine how high up on the roof you want to go with the addition. That is, I am assuming you will tie onto the roof. Most modern houses are too short to tie in under the overhang or from the end of the rafters.
If You do have high walls though, You may be able to remove enough of the roof to actually set the new rafters right on the top plate of the wall framing.
Many options to consider here, A basic idea of what you want it to look like would be helpful.