I think it is very insightful of you to want to learn to be a better critical thinker. That action in and of itself makes me think you are already more of a critical thinker than many others — as merely a freshman you are carefully thinking about and planning what will best help you in the future.
Not the Answer You're Looking For
Unfortunately, philosophy has no real answer to this; it falls more within the realm of psychology and even there you will find no definitive answer yet. The truth is we simply don't know what makes some people better at critical thinking than others. Most will agree that it is a dynamic mix of experience and genetics; the question is how much of each.
Nature vs. Nurture
Take my answer as you will, but in my experience with studying philosophy, psychology, an engineering discipline (cog sci / comp sci), and also from tutoring/mentoring students, I've come to the conclusion more and more that the critical thinking "bug" is about 70% genetic. Either you are born with that kind of mind, or you are not. The rest is experience and personality — personality as classified by Myer's-Briggs typology. I say this because as I look back I realize that it's really something I had all along; I was always questioning things, trying to find the reasons behind events, trying to optimize processes, etc, even from a very young age. Furthermore, when trying to get people to critically think about things (students I mentored or even just people I tried to help in class), I saw that the scientist personality types (e.g. INTJ) tended as a whole to be much better than other personality types, and that little real progress in overall critical thinking ability occurred in a person over time. This would seem to correlate with personality theory, as while personality can change throughout one's life (as in, your typology), it does so gradually, if at all measurably.
So, It's Hopeless Then?
No. On the contrary, while I believe there is some core genetic component, I believe that experience plays a very large role in honing someone's critical thinking skills. Taking Policy and Lincoln Douglas Debate high school definitely helped me learn how to construct strong arguments. Reading tons of philosophy in college helped me learn subtle ways of understanding concepts and dissecting arguments. There are a number of different ways of seeing things that can really open one's mind and help one form strong conclusions better and see flaws in arguments quicker. This is especially true learning to identify logical errors in arguments. Sometimes it's harder to put a finger on the exact way an argument goes wrong than to simply know that it's flawed. These things you can certainly improve upon.
Read lots of philosophy, especially counter-arguments to philosophy you just read. Write your own philosophy papers or book. Pick a topic you are interested in, and write about it. Make a persuasive argument. Have someone critique it (preferably in writing, so you have time to really think about their critique and develop a good response). People here will gladly look at some of your stuff, I'm sure. Just ping us in chat when you have something. :)
Also, be sure to ask your philosophy department professors this same question; this is by no means "the" answer, it is just my experience. :)