There's always more than can be said but I've answered a similar question about a graduate in the early 1900's and this one is about the college admission process. They both get at your question in different ways but the basic gist is you likely wouldn't. You (likely) went to college knowing what your first job was going to be, especially if you were from a family of means.

First, it's helpful to shake off our current mental model around professional employment (grammar school -> high school -> college -> career) and slide things backwards a bit. Basically, if you were a college graduate from an American university in the 1800's, you were likely white, male, and your future was likely set before you started grammar school.

Your father and mother would have most likely picked your grammar school, or hired you a tutor, with your career in mind. For example, if you lived in Boston, you probably would have gone to Boston Latin, a feeder school for Harvard, because you were going to be a preacher or go into politics and because that's where the sons of men with means went before working alongside their fathers. If your father was a businessman in the New York area, odds are you would have gone to a feeder school for Columbia along with the sons of your father's business partners. If you were from a military family, you likely would have been prepared to get into West Point and used family connections to work in the private sector or started a career in the military.

To be sure, there were exceptions and some young men's paths were carved of their own making, but college most a large part of the 1800's, especially before the Civil War, was less about picking a major and getting a job and more about networking and becoming an educated man.