I think you are still of an age when people aren't going to make too many judgments on you for what job you have or even if you have to say you are still deciding on what to do with your life -- 19 is still young enough that this is very common. It's when you're in mid to late 20s I think people start expecting a more substantial answer to that question of what you do.
When someone asks you "what do you do?" or where do you work, some of the things they tend to accept or respond positively to are:
I'm studying to be [fill in blank]/ I'm at college/university at the moment
I'm between jobs at the moment
I'm doing a lot of volunteer work at the moment
Any of the above plus "while I plan my next move" or "while I decide what to do next."
There is also:
I'm an artist/musician/photographer, graphic designer/struggling actor -- or whatever creative field if you are pursuing one of those. If you feel uncomfortable saying that because the pursuit of that goal doesn't actually make you a living yet, you can add "struggling" with a wry smile, or that you are giving yourself a goal to that end "I'm taking time out from work to make more progress with my (acting, artistic goals, etc)
Depending on the situation, some of this may even be overkill or too much information -- as someone else mentioned, sometimes people ask what you do for a living just to make idle chitchat and they really don't care what the answer is, or they hope to make further responses based on your answer just to keep the chit chat alive.
I find it best to keep things brief and then immediately follow up your own answer with a question aimed back at them. "I'm deciding on a new direction at the moment; how about yourself?" / Or: I'm between things/ hoping to take a course -- but I heard you're a vet assistant? That must be very emotional at times."
Sadly, it is true that some people -- maybe even "many" people -- do make all kinds of snap-judgements based on what someone's job is. It seems to be human nature, and it's not fair because I agree with Sweetleaf that WHAT you do doesn't automatically equate with what or who you ARE as a person.
My job has zero anything to do with what I am as a person and who I am, what I actually truly love or care about or am good at. It just pays the bills. But people tend to be shallow.
Just mention studies, college, volunteering, a goal in an artistic field, or even taking a sabbatical to figure out what direction you want to take.
Actually, at 19 most older people don't even expect someone to have figured it all out yet in terms of a career for the rest of that person's life. It's still perfectly acceptable at your age to tell people you haven't yet figured out your ultimate goals but at the moment you are working on that.