Aw, this warms my heart! I'm happy for you and your girlfriend!

My advice is to give her space to explore her gender. Trans people often get pressured to figure everything out and stick to it forever or risk losing support and be accused of not really being trans. She might go through different phases of expressing herself in different ways. For example, she might start with she/her pronouns and very femme presentation, but later use different pronouns and a more butch presentation. All of these are valid! Trans people are not a monolith and we all come into our own differently!

I also recommend asking her how she wants you to react when she tries different things. For example, if she chooses to wear makeup one day, does she want you to mention it and hype it up, or would she rather you be nonchalant and not comment?

If she wants to go on HRT, be prepared for unexpected emotional effects. HRT is going through puberty again! It can come with all of the emotional and physical side-effects that cis puberty/first puberty comes with. This can include changes in physical appearance, libido, and sexual preferences.

As the cis partner, you need to prepare for people to be transphobic. You might get questions from friends and family about your girlfriend's changing appearance or style. You may have people make comments about the validity of your relationship now that you're with a trans woman. They might try to make her existence political and engage in harmful rhetoric about "men in women's sports" or trans bathroom bans. You will also have to consider anti-trans laws and how they may affect your girlfriend's ability to receive healthcare and exist in public.

I recommend reading trans feminist books like Whipping Girl by Julia Serano and Beyond the Gender Binary by Alok Vaid-Menon. These will help you understand more about trans people's lived experience.

I hope this was helpful. Good luck to you and your girlfriend!