Look up the requirements for starting a student group at your school. Sometimes they'll say that you need to have a certain number of group leaders (maybe 2), which can help you decide on who will help you start the group.
Set a first meeting for the group! You can get the word out through flyers, social media, requesting to be included in the school's email list, etc.
Think about what you want the group to do. Do you want to discuss sexuality related topics and learn together, raise awareness on campus, bring in sex-positive educators to have workshops, host educational events, create educational flyer campaigns/zines/etc., push for changes in the way the school is run, build a sex-positive community within your school, etc.? Any one of these (or all of them) would be a great thing to do. Thinking about your focus can help you decide what you want to talk about/do at the group's first meeting.
You don't always need to be an officially recognized student group in order to do stuff. You can often hold meetings, host events, etc. without recognition from the school. This means that you don't necessarily have to wait until the end of the student group application process in order to start your group (and becoming officially recognized by the school may be optional). However, official student groups often get university funding and special privileges (like the ability to reserve rooms on campus).
You can find lots of resources that may help with starting a student group on our Resources for Sexuality Educators & Activists page.
Best of luck! Feel free to contact Nicole with questions at firstname.lastname@example.org.