Well, in short, we're not. We're not supposed to know. Now, that's not to say that if you have things figured out and know where you're headed, that's a bad thing, quite the opposite. However, it is to say we don't HAVE TO know what we want to do, and if you don't know, that's ok.
Most of us haven't lived on our own or held jobs in different fields, and there is a reason many colleges offer a chance to apply undecided. It's because decisions are based on experiences, and most of us haven't had enough experience yet to know what we want to do with the rest of our lives. While I can't give you a foolproof way to determine your college major or career goals, I can give you ways to begin collecting experiences to help you along the way.
For one, internships are a great way to begin trying to figure out what you may enjoy, and there are hundreds of internships - both paid and un-paid - which can help you with that. You can search them out using sites like Indeed or Linkedin, or you can ask friends, family, or companies directly and see if they have any openings. Internships are fun because they can give you a glimpse into what a certain type of job may be like, and they look great on a resume and a college application. Also, internships are a really interesting way to meet new people and network.
Finally, and probably most importantly, IT IS OK IF YOU DON'T LIKE IT. If after you finish your internship, you realize this isn't really something you see yourself pursuing, that's great because at least it helps you know something off the list of possibilities.
Another great thing you can participate in are summer programs. They're fun, they vary in price, and there is always some type of program for teenagers. Depending on the type of summer program you choose, and believe me there are hundreds, you can master a current skill or learn a new one. It'll help you learn about what you may be skilled in, or show you something new that you may enjoy. Another fun part of summer programs is that the participants are teens who are just as curious and unsure as you. It'll help you meet a new community of people, which can be really fun, and it will show you new things.
Also, if you are able to, try a summer program abroad, or even just travel with family. Traveling is a great help because it opens our eyes to new perspectives. It reminds us that wherever we live isn't the center of the universe, and there are so many different ways of approaching things and doing things that we have yet to explore. Learning about new perspectives like this, just like with internships and summer programs, can even inspire us to look at the world with new eyes and change our way of thinking. Overall, it's a way to learn about the world we live in, which is always a benefit when looking at our futures.
Talk To People
You can also talk to people in different industries and get first-hand information about things you may be interested in. I understand if that seems scary or weird, but I promise you that getting first-hand advice from people in an industry is priceless. First-hand advice cuts through the formality of a job posting or an internet search, and gives you real-world emotions and experiences from the perspective of an actual person, and what's more relatable than that. This one is less of a specific event, but rather just a habit that I've found can be incredibly helpful. What's important to remember is that you don't have to start talking to random strangers either. Talk to a family friend or a really nice teacher, just someone who has some real-world experience they wouldn't mind lending you.
Lastly, don't entertain naysayers. If someone is not supporting you, but you believe you can do this, then ignore them and have faith in yourself. I know that's not as easy as just waking up one morning and deciding, but it is worth it. Also, please remember not to limit yourself. If you like more than one thing, do both. It is possible, just look at the incredible multi-tasking peeps with talents in multiple fields. You can do it too! So please believe in yourself, stay true to yourself throughout this journey, and don't put yourself on a timer. Everybody learns what they like at their own pace, and that's perfectly fine.