When you think about what your step-father has done to you, how angry are you on a 1-to-10 scale? When you think about your Mom and what she has done to you, how angry are you on this same 1-to-10 scale? I recommend that you start with the one person who gets the lower score, the one toward whom you are less angry. I suggest this because it can be difficult to forgive when you are fuming at a particular person. Learning to forgive when you are less angry helps show you the path of forgiving and gets you ready for the more challenging one.
Learn more at What is Forgiveness?