Mary, the basic answer here is 45 degrees to make square corners, but often corners aren't quite perfectly square.
It's a good idea to have a couple scraps (12" long each is good) cut to a 45 degree angle (Left & Right) you can use to "test" the corner first by holding them in place and checking the joint. If they fit tightly, cut your pieces at a 45. If the test joint is slightly open at the front, adjust your cut angle lower by a degree or two (44 or 43). If the joint is tight at the face, but open at the back, use a higher degree of angle.