High school is going to be an absolutely crazy beginning of your life. This is when you find out what you want versus what you don't want in many different aspects. However, this doesn't mean you have to know everything now. You don't have to have all the answers. If I'm being honest, you don't even have to have half of the answers.
Plans will change throughout high school as much as you will, and that's not a bad thing. It's a little scary at first, but remember confidence in yourself will help guide you through any obstacle you think of facing in a high school environment. Being confident is a tough task on its own as well as self love, and will have to be worked on each day. Being okay with how frequently things change and being able to “just roll with it" will help relieve stress.
This being said, a lot of emotion will be put into high school. Realize school is going to be a large part of your life because it's the majority of your time. You will feel pride and disappointment from time to time and that's natural. Either way, you will become familiar with your school and these up and downs, it makes you feel whether you want to or not.
Be as involved in your classes, extracurriculars, and school activities as possible. Being involved will help build relationships whether it's with a first romance, a couple friendships, or even being accidental teachers pet.
Don't rush the process. Finding new friends or interests will take time. It can also be extremely difficult at first. Don't feel bad if you don't click with someone or something right away. Pushing yourself out of your comfort zone, even by a tiny bit at a time, can be a large feat itself. You're just beginning a new journey.
Keep in mind that you're not better than anyone else. In the same sense, no one's better than you. You're all going to school for close to the same reasons. If you don't think you're cooler than everyone else, people will tend to like you even more for the humility. Confidence and cockiness are two completely different things and popularity is lonely.
In your new relationships that come naturally through high school, remember that everyone has a story. Be cautious and respectful with the information you're given. People don't have to tell you anything about themselves and vice versa.
You're not going to see many of these people after high school. That doesn't mean say whatever you want or do whatever you want, but don't be afraid to be yourself. You are each your own person with your own interests and you should never be ashamed of it.
Put 100% into everything you do. Being open minded, working hard in class, shooting a basketball hoop, reciting a poem, ANYTHING. Just make sure you don't leave thinking “I could have done more." Leave your mark on high school and take it slowly. It can be extremely thrilling if you decide to take your time and enjoy the experience. Good luck!