This is a good question! And a lot of it takes asking yourself a lot of questions.... :)
The first questions should be "what do I like to do?" and "what am I good at doing?" A career is a path that will involve a lot of your life... and if you don't like what you are doing, it is very wearing and makes it hard to do.
You should also ask "what is in this area where I can make a living?" This is something that you have to do any in any case. You may like to make clothing... but are you good at it enough that people will pay you to be a tailor?
Also be realistic. While it would be fun to be a "movie star" or "rock idle" or "famous sports star", there are VERY FEW of those in the world... so the chance of you becoming one of those is also small. And it is often unnecessary... the vast majority of people never get "rock star" status and live happy lives. If you like medicine, but don't think you will high enough marks to be a doctor, you might be a good nurse. Or assistant. Or helper in some other way in that field.
Your education and your skills are the tools for your career. Make sure you have as many of them as you can and they are as strong as possible. EVERYONE should know something of economics and business, for instance... you are a consumer and will work for a business (or maybe run one!). Everyone should be able to communicate to others and understand what they are saying to you. Being able to receive instructions and give information is pretty basic in any job. And the better you are at any of this opens doors in any career.
Don't be afraid to dream, but also be flexible. Realize that your path will probably not go exactly as you planned, but it is likely to be a good path if you let it. You will have good points and bad points, and grow as you go.
Best of luck. Explore and enjoy exploring!