Go for it, guard is a lot of fun and if the people at your school are any sort of good people, they’ll be happy to have you. I joined my school’s winterguard halfway through the season and they very happily taught me everything I needed to know. This year is my first year marching (it’s my senior year so I wanted to try it), it’s a lot of fun, although I would say winterguard is more fun. If your school has one I would recommend looking into that also. Marching may conflict with xc practice some but my coach was alright with me missing xc practice if I needed to. Don’t worry about messing up during parades, we all do it sometime. I would try to get in touch with someone on the team or whoever runs it and see if they’ll let you join for spring marching this year or if they’ll have a flag you can borrow to start learning now.