I have known since I was a kid....assuming of course I keep going in my current field after I finish my PhD.
I would recommend sitting down and figuring out what you *enjoy* doing. This should help to guide what you do for a living. I think a common mistake is for people to go after things they *like the sound of* (e.g. doctor, lawyer, etc.), but in practice do not really enjoy spending the time to do it well. You always be best at doing things you *really* enjoy. In theory, that suggests you will be the most successful at doing something you truly enjoy.
With this in mind, I do not thik it is always the best idea to get too hung up on "careers". A lot of time peoples' real interests do not have a pre-defined career path to go with them. In order to support yourself (and your family if you have one), all you really need is something to sell to other people, whether it be a skill, talent, experience, etc. And I think it is not uncommon to choose to try to sell something that other people also commonly sell - in other words, something other people have shown time and time again people will buy, is therefore marketable and can provide a good living.
But that isn't to say you can't put a good sales pitch on something unorthodox. I think a good example is personal training. We have had farmers, bakers, carpenters, etc. for hundreds or even thousands of years, but we have only had personal trainers for a few decades. A few decades ago, somebody decide they like to train, and that they will try to sell that experience to other people.
Anyways, the above route of putting a sales pitch on something unorthodox obviously depends on what sort of person you are, and whether or not this sort of thing interests you. But I think it is something to keep in mind and can ultimately lead to long-term happiness, albeit that sort of venture usually requires a lot of work to initially get off the ground.
Alternatively, knowing how you enjoy spending your time will still help to guide you in choosing a more standard career. And once you do that, it is usually pretty standard what course of action to follow - school, internship for experience, etc. There is usually a standard for the field and obviously some require more work/training than others.