Don't wait for her to ask you to do things. Between having to deal with her changing body, the surge of hormones, and all the questions that come with giving birth and becoming a mom, she's not going to always be able to express how to help and support her. Dad's Pregnant, Too! helps men define their own roles and figure out how they can be proactive and support their pregnant partners without her having to ask. A lot of times, she won't even know how you can help.
Have a birth plan. Women giving birth need their partners to be advocates. The only way a man can be an advocate is to know what's normal and to be expected. We have much more control than we think.
Don't worry until you have to. Trust that it will be the most amazing, incredible, and powerful experience of your life. Until you have reason to worry, don't.