Pursue your degree (and the knowledge and skills it entails).
Be skeptical. Do not accept or presuppose "privileged knowledge". Don't accept things on authority --- accept them (provisionally) because there is reason and evidence to do so.
Strive for intellectual honesty. Don't bend observations to support your favorite notion(s).
Understand the hypothetico-deductive model and its role in the creation of knowledge.
Try to apply the multiple working hypotheses (Chamberlain) and the strong-inference (Platt) notions. They may not always seem to work in their strong form, but identifying alternative explanations and then planning and making the observations that can assess their consistency with our understanding does tend to advance our understanding.
Understand that advocacy is not research. Particular causes or policies may be valuable to you, and even to society, but assuming them and then arguing for them, is not research.
Learn to program computers. I suggest Python as the language to start with these days, though cases could be made for several alternatives. The point is to have a tool for exploring and thinking.
Read. Start with the list below. Augment it with materials in your interest area. Keep notes on your readings and thoughts they provoke. Use a text file, bibliographic software, 3x5 cards, loose-leaf notebooks, composition books, or some other mechanism, but keep notes to remind you of what you've read and thought. Scan back through these notes from time to time. Particularly note the questions that come to mind, and speculations on ways of answering them. Working over some of this common intellectual ground will benefit our discussions, your discussions with others, and may even be of value in preparing for things like thesis proposals and comprehensive exams.
Abler, Adams and Gould. 1971. Spatial Organization: The Geographer's View of the World. Prentice Hall. Englewood Cliffs, NJ.
Bunge, Wm., 1966. Theoretical Geography. Gleerup. Lund.
Chamberlain, Thomas C., 1890, "The Method of Multiple Working Hypotheses", Science, v. 15, n. 92. (Reprinted in Science, v. 148, May 1965, p. 754-759.)
Chrisman, Nick. 2006. Charting the Unknown. ESRI Press. Redlands, CA.
Felleisen, M., Findler, R.B., Flatt, M. and Krishnamurti, S. 2001. How to Design Programs: An Introduction to Computing and Programming. MIT Press.
Fisher, Peter, and David Unwin, eds. 2005. Re-Presenting GIS. John Wiley & Sons, Ltd. The Atrium, South Gate, Chichester, West Sussex P019 8SQ, England (but there's probably a US issued version too).
Haggett and Chorley. 1967. Models in Geography. Methuen. London. (Who ever kept my copy, please return it!)
Landauer, Thomas K., 1995, The Trouble with Computers: Usefulness, Usability, and Productivity. MIT Press. Cambridge, MA.
Manicas, Peter. 2006. A Realist Philosophy of Social Science. Cambridge University Press.
MacEachren, Alan M., 1995, How Maps Work: Representation, Visualization, and Design, Guilford Press. New York, New York.
McMaster, R.B. and Usery, E.L. (eds). 2005. A Research Agenda for Geographical Information Science. CRC Press. (Hamilton: G70.212 .R47 2005)
Norman, Donald, 1989. The Design of Everyday Things (alternatively sold as The Psychology of Everyday Things 1988.)
Norman, Donald. 1992. Turn Signals Are the Facial Expressions of Automobiles
Pavlidis, Theo. 1982. Algorithms for Graphics and Image Processing. Computer Science Press. Rockville, Maryland.
Peucker, Thomas K. and Chrisman, Nicholas R. 1975. "Cartographic Data Structures". The American Cartographer. 2:55-69.
Peuquet, Donna. 2002. Representations of Space and Time. Guilford. New York, NY.
Platt, John R., 1964, "Strong Inference", Science v. 146, n. 3642, 16 October 1964, p. 347-353.
Preparata, Franco P., Michael Ian Shamos. 1985. Computational Geometry: An Introduction. Springer-VerlagNew York.
Robinson et al. Elements of Cartography
Samet, Hanan. 1989. The Design and Analysis of Spatial Data Structures. Addison-Wesley. Reading, MA.
Samet, Hanan. 1989. Applications of Spatial Data Structures: Computer Graphics, Image Processing, and GIS. Addison-Wesley. Reading, MA.
Shneiderman, Ben 1987. Designing the User Interface: Strategies for Effective Human-Computer Interaction. Addison-Wesley. Reading, MA.
Worboys, Mike. 1995. GIS: A Computing Perspective. Taylor & Francis. Bristol, PA.
The American Cartographer / Cartography and Geographic Information Systems / Cartography and Geographic Information Science
Cartographica / Canadian Cartographer
IJGIS International Journal of Geographical Information Science (Systems through 1995)
Environment and Planning A
Environment and Planning D
Geographical & Environmental Modeling
Transactions in GIS
Computers & Geosciences
J of Spatial Cognition and Computing
Other journals pertaining to your specialization should also be on your list.