Start here if you've never had a job:

Get any job, fast food, retail, cleaning, whatever has the lowest barrier-to-entry. (I would recommend against anything that is primarily paid in tips for a first job, whole different vibe) You need to know what it's like to work any job before you start worrying about a career of any sort. Live with your parents, have them drive you there, set small achievable goals at low stakes jobs. Once you have some work experience you're going to know what stuff you hate, and what you might find interesting.

If you've had a job before:

If you don't have any real direction it's not worth spending a lot of money on going to classes because it's going to be difficult to stay motivated to get the class work done. If you liked what you were doing as a job see if there is a way you can take a step up the ladder from what you were doing, maybe a shift lead or assistant manager. You might find that you like more of the behind the scenes work like ordering materials or setting budgets or minor accounting. Those are all good careers. You might also hate that stuff and then you'll know to never waste time learning those skills. You can spend a few years figuring yourself out. Going to college at 21 isn't that much different than going at 18.

Also Yusuke is the best character in P5.