How can one chose any job? By making oneself familiar with that job and the alternatives. A good way for that is an internship. The second best thing is reading how professionals describe their job, watch documentaries, and talk to the pros.
For example, if you are love preparing food and like to bake, cook and such, then it would be a good idea to spend one summer in a restaurant kitchen, another in a bakery, a third in a factory that mass-produces foods, and so on, and then choose wether you want to be a cook or baker or study food technology or something similar.
Of course many kids don't do that, and, as we know, as a result many adults are more or less frustrated by their jobs. So its a good thing you try not to be one of them.
My advice would be, as you can guess, to start by making a list all the possible jobs a writer can have. From ghost writing pulp magazines, to scripting tv shows, to writing computer game storylines, to writing comics, and so on. Also consider the topics you are interested in. Are you interested in writing political stories set in the real world and actually influencing how people vote? Or are you more interested in zombies eating children? Not all genres fit into all kinds of writing, so this might give you a first limitation.
Then start do look into how the daily life of the people writing all that stuff looks like. There are blogs by writers, books on writing and writers, magazines for writing professionals, documentary movies and so on. Grab as much as you can and get a first idea of how these people live (and wether or not they actually earn enough money to have only that writing job, of if they have to dive a taxi at night or be a teacher during the day). Not all lifestyles will fit you, so this might give you another sub-selection.
Now try to contact people in the areas you are interested in. Seriously, just write an email, go to a reading, or whatever you can do, and ask everything you have not yet understood. Be polite and don't bother them when they signal they have no time, but some writers are readily available in forums, on facebook, or even in person on a convention. Don't be shy, writing is a lonely business, and many writers love to socialize. Others don't, so don't take it amiss if they are no time for you.
Depending on what areas you find most interesting, see if you can find a company doing that stuff. For example, if you are interested in computer game writing, find a computer game company in your area and ask them if you can spend a few days with them. Or if you are interested in writing for mangas, find manga writers in your area and ask them if they would allow you to visit them for a day or two. Networking is a necessary skill as a writer, so you can start learning to be sociable and make contacts. Go to conventions, workshops, etc., and meet people. Don't be a basement dweller.
Note: All writers hate it when amateurs ask them to read their writing, so do not ask any writer to read your stuff. They won't, and it will show them that you are not yet mature enough to work in the field.
Finally, try to write what you think you want to write. Get books and stuff that teach you how to do it. Writing books may be obvious, because you know what a book looks like, but what does a script for a manga look like? What is the process from the idea to the finished product? Inform yourself. And then try to do it. You may find that you don't actually like to write what you like to read, or that you like to write what you don't actually read much.
Take your time with all of this. I don't know where you stand in life, but I would advise you to take a year and do all of this as a kind of hobby besides school or university or your job.
Know yourself and know the diverse field of writing, and then decide.
Have fun and good luck!