First off, kudos for starting to take a look into your future at such a young age! And I am happy to hear you are a very positive and hard working young lady! (Also, your English was fine! Better than most native speakers I encounter!)
I'm going to drop you into a soft pillow of reality for a minute, from an unbiased perspective.
1. You are never to young to start thinking of what college you would like to attend in the future and you're about the age where by putting in the hard work it will really start to pay off! I suggest becoming a working student at an accredited facility so that way you learn both ends of the horse world; the dirt and grime of the barn chores as well as the fun and challenging riding/training/ground work aspect. By doing this and advancing in your riding you could look into riding scholarships to help students through college. Some colleges will even personally offer scholarships to riders who apply. Always, always continue to look and apply for scholarships (normally your last year of high school (or your equivalent)) as they will most definitely help towards paying your tuition. I know 10-13 thousand sounds like a lot of money right night but it goes quickly. Another thing you can do is to start setting aside money now be it $5 or $30 a week in envelopes or given to a trusted family member this can be your food, book, and living expense money when you get into college. This will help you a TON believe me, this is probably what saved my behind when I was in college (non horsey major but we'll get to that later) Also, I know in the US we have something called a FAFSA which offers financial aid help to those who qualify look into seeing whether or not Puerto Rico has something along those lines. It helps with grants, loans, scholarships, work study courses, etc.
2. NOT a stupid question at all, the only stupid question is the one not asked. Some colleges will have you apply and if you are accepted you are to then take their entrance exams to see what level of course you would be suitable to start out in. Others have you take exams depending on the program that you are entering. For example, someone looking to become a theatre major may have to audition before they can enter the school. If you are looking into doing an equine major position that includes horse handling or riding they may make you send an audition tape or take a riding exam to ensure that you have the basic level of skills to enter and be successful in the courses that you are interested in. This is where that working student position will come in handy.
3. Yes and no about your expectations as life has a funny way of keeping you humble. Keep your dream and work at it, work hard. But may I suggest that there is not much that one can do with an equine degree so maybe major in business management with a minor in equine coaching/training. That way if life throws you a curve ball you will have your non-equine degree to fall back on should you need it. The business degree will also help with the coaching as you would be essentially running a business and your business degree would give you a real edge in the equine world because many people can be coaches but not as many people know how to run a business properly AND coach. As for being a professional show jumper be prepared to WORK HARD if you really want that. That working student position I mentioned comes in handy again. Study, practice, work hard, work under multiple trainers to learn various styles. This is how many pros start out by apprenticing under people who already know what they are doing. It is a HUGE sacrifice of your time, energy, money, and resources but if you really want it, it will all be worth it. Still the non-equine degree to fall back on should help a lot as a VERY small percentage of people who are trying actually make it to the Professional level.
4. I have mentioned a lot about the back up plans in the above answers. I would definitely major in a non-equine course first and minor in the equine studies it just gives you a nice buffer should the horse world not work out in your favor.
5. Unfortunately, I did not go there so I cannot answer this one, my apologies.
You are not setting your expectations too high at all it's just whether you are willing to put in the amount of work it takes to get there and then risk the chance that it will all fall apart once you get there because unfortunately nothing in this life is guaranteed.
And please stop calling yourself dumb, the fact that you are looking forward to your future and doing what you can to be prepared proves that you, my dear, are far from dumb. I had to stumble through the world of college alone because no one in my family had ever done it and as much as I felt "dumb" at times you keep your chin up, ask questions, and hold onto what you learn as it will help you in all aspects of your life. Don't put yourself down or mock your psyche ever, you are doing amazing! Keep checking around to different colleges to see what they all have to offer. Some have equestrian courses as well as regular courses, some have equestrian teams, some have just equestrian studies; there are so many different things out there that you really have to part the reeds and take a good look around!
Good luck! And the horseforum is here for you to try and answer any questions that you may have! Never stop asking questions as it is the best way to learn! ~hugs~
~ Hope is never light years away ~