Hi Ruby, I run a team for your age group, and I'm a coach. What I'd highly recommend, is to find your nearest local club which has Youth coaching. You will be in the 'Youth A' (under 16) age category. As you begin, the best thing you can do is to just get out there and practice. If you're interested in competing, you'll find lots of advice, training and coaching available at little to no cost when you join the right club.
At your age, races are only 25-50 minutes long, so you're best working on speed endurance, rather than pure endurance. If you get into cycling, and do well, you'll find there's a lot of support and resources at the right club, so do look around, and try to find one with a team that you might be able to join, and good training provision.
If you don't want to race, but just want to do long rides, then still join a club, but you don't need to worry so much about training provision, and can join social rides. I would highly recommend that you get all the training you can though, to improve your cycling skills. There's a massive amount of difference between being able to ride a bike (whether over a long distance of short), and being a skilled cyclist, with great control over your bike, ability to handle it really well, potentially avoid accidents/ crashes. You're still young enough to learn how to cycle really well.