The first thing you can do to help your daughter is to learn about female puberty in great detail. When she does come to you with questions, you'll need to know what you're talking about. The best time to educate yourself about female puberty is before your daughter has started going through it. The second key step you can take (also ideally before puberty begins) is to enlist a trusted woman who can be the female resource for your daughter. If her mother is still part of your daughter's life, discuss with her how you can handle your daughter's puberty together. If your daughter's mother is not an option, you can look to a female relative as an alternative. Keep in mind that both you and your daughter must trust this woman.
When your daughter's puberty does begin, let her know she can come to you with questions and concerns. Even if she won't come to you about everything, it will still be important to her know you care and still accept her as she goes through all these changes. Some important specific notes: Menstruation is normally the scariest part of a girl's puberty, so make sure that your daughter is taught about it before her period begins. Since her hormones will start producing greasier hair and skin, you should also make sure she starts showering daily. Puberty is often when girls start comparing their bodies to other girls, and they can start to feel embarrassed and uncomfortable about their bodies. Understand that your daughter is likely to feel this way, so you don't want to validate those feelings by showing your own discomfort.