1) It will be pretty hard to take out a wall without some damage to the ceiling. To install the beam youíll first need to build a temporary wall to support the joist while you installing the new beam. Be sure the floor under the temporary wall can support it. As for the beam, check with the building inspector or an architect to get the right size. It sounds like you may need something like a micro-lam beam.
2) I assume you will be adding rafters off the existing roof at a different pitch. You should remove the shingles for sure, but you donít really need to remove the sheathing (the plywood). You could install the new rafters directly on the sheathing but it would be better to first install a cleat (a 2x12 or whatever) flat on the sheathing across the existing rafters and install the new rafters off the cleat. After installing the cleat, cut away the plywood below to allow for ventilation from the new soffit vents (yes, install soffit vents) to the old attic to the ridge vent if there is one.