Don't worry, you'll be able to renew the brakes just fine on your own. Its one of the most basic repair skills to learn. And best of all, you have this forum which was unimaginable when I was your age. When I was 15, my father traded for a wrecked, late model Honda Civic with a damaged motor. He trailered it home and surprised me with it one day after school, saying (and I quote) "Son, if you can fix it, you can drive it."
Only in my time, Supras were brand new cars and the internet hadn't been invented yet. I had to go downtown to the library and check out a Chilton's manual and other books on car repair in general. Dad took me shopping at salvage yards and sometimes supervised when I had to do something relatively dangerous, like hoisting the motor out with a rented shop crane, but otherwise he barely lifted a finger to help. I had to spend my savings to buy a whole front clip, motor and all off a similar Honda that had been smashed in the rear and have it painted to match. Took me all summer. But looking back on the experience, if I were POTUS, I'd sign an executive order that every teenager has to get their first car the same way I did. Wow! What a fantastic way to educate a young driver about everything he really ought to know about his car and make him drive it carefully, having so much skin in the game!
So kudos to you (and your cousin) for wanting to jump in and get grease under your nails. It might not be the start of a lifelong car hobby like it was for me, but even if after you graduate from college or trade school and you can afford to always have a new car under warranty or pay those professionals, you'll at least know enough about how cars work that the mechanics can't easily overcharge you for repairs or talk you into service that isn't necessary.
There's lots of information on this forum specific to the Supra brake system and you can find "how-to" videos about servicing brakes on youtube. But I'll give you some warning about working on 40 year old hydraulic brake systems. Most car owners seem to know how often they are supposed to change engine oil, but almost none know that the brake fluid is supposed to be changed every 2-3 years. I wouldn't be surprised if your Supra's brake fluid is decades old and there is a lot of internal corrosion. If the car didn't come with maintenance records, I would assume the worst and plan on replacing the master cylinder and calipers with new or remanufactured units. Tho looking at current availability on rockauto.com, you may have to delve into getting the seal kits and rebuilding the calipers yourself. But no worries, its not hard, we can walk you through it if you get stuck.
