I'm thought about it and I plan to tell my dad how I really feel about my step-mom.
Is there a reason that you want to tell your dad?
Because I have so much anger in my life.
It's fine to be honest with your parents. If you have these feelings of anger then sometimes sharing them is the only way to come to terms with how you're feeling.
I don't trust my dad and I think he'll get mad and storm out.
I think it's brave of you to share your feelings with your dad and I understand the fear about his reaction. Try to approach the conversation calmly and with maturity and hopefully your dad will control his reaction and listen to your feelings.