MathematicsIt is absolutely essential that a physicist be proficient with mathematics. You don't have to know everything - that's impossible - but you do have to be comfortable with mathematical concepts and how to apply them.
To study physics, you should take as much high school and college mathematics as you can reasonably fit into your schedule. Especially, take the entire run of algebra, geometry/trigonometry, and calculus courses available, including Advanced Placement courses if you qualify.
Physics is very math intensive and if you find that you dislike mathematics, perhaps you will want to pursue other educational options.
Problem-Solving & Scientific ReasoningIn addition to mathematics (which is a form of problem-solving), it is helpful for the prospective physics student to have a more general knowledge of how to tackle a problem and apply logical reasoning to arrive at a solution.
Among other things, you should be familiar with the scientific method and the other tools physicists use. Study other fields of science, such as biology and chemistry (which is closely related to physics). Again, take advanced placement courses if you qualify. Participating in science fairs is recommended, as you will have to come up with a method of answering a scientific question.
In a broader sense, you can learn problem-solving in non-science contexts. I attribute a lot of my practical problem-solving skills to the Boy Scouts of America, where I frequently had to think quickly to resolve a situation that would come up during a camping trip, such as how to get those stupid tents to actually stay upright in thunderstorms.
Read voraciously, on all topics (including, of course, science). Do logic puzzles. Join the debate team. Play chess or video games with a strong problem-solving element.
Anything that you can do to train your mind to organize data, look for patterns, and apply information to complex situations will be valuable in laying the foundation for physical thinking that you will require.
Technical KnowledgePhysicists use technological tools, especially computers, to perform their measurements and analysis of scientific data. As such, you need to be comfortable with computers and different forms of technology to. At the very least, you should be able to plug in a computer and its various components, as well as know how to maneuver through a computer folder structure to find files. Basic familiarity with computer programming is helpful.
One thing that you should learn is how to use a spreadsheet to manipulate data. I, sadly, entered college without this skill, and had to learn it with lab report deadlines looming over my head. Microsoft Excel is the most common spreadsheet program, although if you learn how to use one you can generally transition to a new one fairly easily. Figure out how to use formulas in spreadsheets to take sums, averages, and perform other calculations. Also, learn how to put data in a spreadsheet and create graphs and charts from that data. Believe me, this will help you later on.
Learning how machines operate also helps provide some intuition into work that will come up in fields such as electronics. If you know someone who's into cars, ask them to explain to you how they run, because many basic physical principles are at work in an automotive engine.
Good Study HabitsEven the most brilliant physicist has to study. I coasted through high school without studying much, so I took a long time to learn this lesson. My lowest grade in all of college was my first semester of physics, because I didn't study hard enough. I kept at it, though, and majored in physics with honors ... but I seriously wish I'd developed good study habits earlier.
Pay attention in class and take notes. Review the notes while reading the book, and add more notes if the book explains something better or differently than the teacher did. Look at the examples. And do your homework, even if it's not being graded.
These habits, even in easier courses where you don't need them, can help you in those later courses where you will need them.
Reality CheckAt some point in studying physics, you will need to take a serious reality check. You are probably not going to win a Nobel Prize. You are probably not going to be called in to host television specials on the Discovery Channel. If you write a physics book, it may just be a published thesis that about 10 people in the world buy.
Accept all of these things. If you still want to be a physicist, then it's in your blood. Go for it. Embrace it. Who knows... maybe you will get that Nobel Prize after all.