Feeling anxious before your first therapy session is completely normal. It can be intimidating to open up to a stranger.
Remember that the therapist is a trained professional whose job is to create a safe, non-judgmental space for you. You don't have to dive into your deepest traumas right away. The first session is usually about getting to know each other and discussing your goals for therapy. You can even tell the therapist that you're feeling anxious - they will understand. Just showing up is a huge step, and you should be proud of yourself for taking it.