Your role in your baby's birth doesn't start when you walk into the maternity unit. It starts during pregnancy. Preparing yourself doesn't mean learning a list of things you will have to do during the birth.
It starts with something much simpler: talking to each other. Ask her how she imagines the birth, what makes her feel safe, what worries her, and how she would like you to be present. Some women want plenty of touch and encouraging words, while others might need silence, space, or simply to know that you are there.