I am 15 years old and have 3 years to go before I go to the university, what should I do to prepare myself for the university?
Easy. Get the best grades you can. Do the best on the SATs you can. Take the hardest classes you can in your high school.
Start practicing your essay writing.
Start doing extracurricular activities if you are not already, but I believe leadership in one counts for more than membership in many.
Excelling in a sport might help, but that it kind of out of your control.
Notice I’ve been talking about things to do to get into college. If you do all this stuff,
actually being in college will be a snap. The only real difference is that you won’t have parents yelling at you to do your homework or to go to bed.
I suspect this thread will be in IMHO soon, so look for it there.
The OP is not from USA. So, I really don’t know if our (U.S.) experience is transferable. But I’ll contribute anyway. Rule 1 ,when in doubt, don’t worry too much.
Work on your social skills. College is the one time you have to meet a huge number of people your own age who are actively looking for friends, dates, and group social experiences. Further down the line, those friends can become the heart of your network of people who would want to help you find a job. That chance doesn’t come again if you miss it.
Shy/socially anxious? Get help now, don’t wait. That doesn’t mean drugs, it could mean social coaching. if you are on the autism spectrum as you alluded in your other post, you should be particularly concerned about your social skills. But this advice goes for anyone. College is valuable as a social experience as much as it is an academic one. If you are not making friendship connections and growing in your social abilities, you’re not getting everything out of it that you can.
Similarly, any issues related to executive function (ability to organize & prioritize tasks) should be addressed now, so that you are able to self-organize your studies as college generally requires.
Then it depends strongly on the country. My daughter has a German boyfriend who messed up on his exams at an early age, and once maturer had to work himself up to a decent college by a very roundabout way. She had a previous English boyfriend who did well on his exams and got into college with no problems.
But in any case good study habits in high school translates into good study habits in college.
There is one little danger - my roommate at MIT was so much smarter than anyone his teachers in high school at Texas ever saw that he got As with almost no effort. Then he hit the wall. I was lucky in having a lot more competition even in high school.
Find something you’re passionate about, and pursue it. I got perfect grades and excellent SAT scores, and was a member of a number of clubs, which got me into a some good schools, but not every school. A good friend of mine got good grades and ok SAT scores, but he was deeply passionate about carnivorous plants. He built his own greenhouse in his backyard, cultivated plants, learned everything he could about them, and became an expert on them.
He got into every college he applied to, and ended up going to an Ivy League school.
Grades and SATs are important, but only to a certain point. Finding something you care about and can demonstrate not just participation in, but excellence at, will set you apart from other candidates.
That advice goes for college and the rest of your life, too. Go ahead and let your grades slip a little bit and try doing something different and inspiring. No one cares about your GPA as long as it’s “good enough”.
Since the OP is looking for advice, let’s move this to IMHO.
Colibri
General Questions Moderator
Well, my dear compatriot (if that really is your last name ;))… you don’t need to do much. Just make sure you have the required ‘profile’ and you’ll be prepared just fine. Most of what you cover in High School will be old news after the first couple of weeks, so it’s far more important what attitude you show once you are there… unless you want to go into medicine off course, as you need a high GPA to evade the lottery.
If you’re going away from home, then learn how to do your own laundry, sew on a button, etc.
Also, learn some self-control with respect to food and drink, especially the latter.
Start practicing time management, it will help you a lot when you have multiple classes with multiple assignments and you need to figure out when you can get everything done. When I was in college I always bought one of those big planning calendars and put everything from the syllabuses on it in a different color pen (one for each class). I can’t tell you how many times my classmates “forgot about” an upcoming quiz/text/paper and didn’t enough enough time to prepare. After awhile they started asking me if anything was coming up that they should know about… 
Start trying to live independently of your parents. Earn your own money and buy your own things. Do your own laundry and cook your own meals. Many students can’t hack it in their first year because the adult world was too new and too foreign for them to handle.
Some good advice here. I will add: look at the colleges you might want to attend and check their entrance requirements. Ensure you are on track to meet those.
At my high school, science classes were electives. I didn’t take chemistry in my junior year. When I found out most colleges look for that, for my engineering major, I had to take high school chem during a summer session between junior and senior years, and then physics during my senior year.
Depends on what your goals are. Either study hard, or buy a bong and work on developing a taste for beer.