Meh. A mini needs just as much care as a big horse- hooves, regular vet exams, etc. and you can't even ride them. They are loads of money to pay for, just like a normal horse, and unless it will live in your yard, you'll need to pay the barn fee. (300/month or so) Minis are also prone to health problems and can be nasty little buggers sometimes too... I'd stick with the big guys unless you are absolutely bent on having a mini. If you are, I'd get a gelding. Make sure it has a good temperment and confo- I wouldn't spend more than 500 on one. As to vet costs, I'd set aside at least 1000/year just for safety's sake. Think it through, please, before you make a decision. Any horse is a major committment and it will rely on you for ansolutely everything. For its sake, please think it over. Are you 100% sure you can handle the responsibility? If not, I'd wait a little while.
As to making money, I do believe groceries hire part time employees ages 14 and up. You can do odd jobs around the neighborhood until then, or maybe start a pet sitting service if your parent are agreeable.