No matter what you choose, build stuff and be around smart people. “Stuff” can be a lot of different things—open source projects outside of class, a startup, a new sales process at a company you work at—but, obviously, sitting around talking with your friends about how you guys really should build a website together does not count.
100% agree with what Sam is talking about here. His advice applies to much more than 19 year olds, really to anyone at the beginning of their career.
Working with smart people on awesome stuff, whether it’s from 9-5 or outside is the crucial point here: you get better at what you practice.