Convince your parents to let you get one as you will need their help and their commitment too! I have recently bought my own horse at 17, with a driving licence, working two jobs, full time education and it is completely draining! I love it but it is very hard work! Thankfully my mum helps out and I wouldn’t be able to do it without her!
A horse is a very long investment. They are not cheap, and emergencies do happen (a lot). Some yards will let you have DIY livery in exchange for work, but that doesn't help with vet/farrier etc bills. Rescues are an even bigger commitment as you do not know their past, their potential medical issues and their potential behaviour issues. Unless you have a job and your parents are willing to help in emergencies, I would hold off until you can afford it. Don't forget you still have to pay for your horse through college.