How does one become a big brother or big sister?

Recently I’ve become very interested in the idea of becoming a big brother/sister and mentoring someone. Does anyone have any personal experience with this? What are the requirements? I’m 23 and think I can really help some young ladies not make the same mistakes that I’ve made. I’d like to reach out to high school kids too. Does anyone know what the process would be like or what kind of candidates they want? Thank you so much.