The only tips I can give are to get your parents involved and make it clear that you're prepared to pay a contribution and do stable duties on the days you ride. People are understandably reluctant to enter into an agreement with a 15yo because if there's a problem they have little or no comeback and could also have parents on their back. Also they will worry that schoolwork will get in the way of horse care, and how you will get to and from a yard. Plenty of yards won't allow under 16s unsupervised which is another problem. You really need your parents to help you look and contact people looking for sharers. Also be realistic about share ads you reply to, it may not be what you want to hear but you need to be replying to ones that are quiet rides, suitable for a novice.
You have to understand that peoples horses are precious to them so it's understandable that they want someone who is experienced in riding and horse care. Do you have any experience in handling and caring horses? If not that's something you may want to work on by helping out where you currently ride. Also a lot of people, and I include myself in this group, don't want their horse ridden or handled by a novice who may mess up it's training or be unable to deal with problems which then escalate.
I'd say get more experience at your school, and also look to help out there to get experience handling. If there's a yard near you you could also offer to help there, but don't assume that will automatically get you a ride and again you may need your parents to come along and meet the yard owner and maybe sign something to say they agree to you helping and don't hold the yard owner responsible if you get hurt - horses will be horses.