The best advice I can offer to you is to document your interest in advanced mathematics through courses and standardized tests. Understanding advanced mathematics and having a thirst to learn more is admirable, but it will only cross over from being a hobby to being something colleges will take into account if you show what you've learned.
Taking higher level classes at your local college is a great start, especially if the classes are ones that require a certain level of baseline knowledge as a prerequisite (e.g., multivariable calc requires having a solid grasp of calculus beforehand). You might also consider online open course modules like EdX or Coursera, particularly if the schools you're thinking of applying to have courses on there.
A couple of other things you can do to demonstrate your passion for advanced mathematics are to excel at every math standardized test you can (SAT Math 2 Subject Test, AP Calc AB test) and to take part in math-related extracurricular activities like the Math Olympiad.