Framing a Wall with a Sloped Ceiling
You need to cut your top plate pieces into parallelograms that match the angle of the ceiling. If you are framing the wall with 2x4’s then you’ll need 2x6’s to cut from, or maybe even larger, depending on the angle. A steeper ceiling will require a wider parallelogram. 

You can cut these top plate pieces easiest with a table saw or with more effort a hand rip saw.