Take the time to figure out what you're really interested in. Follow your interests without regard to where you may wind up. Start reading and thinking hard about your subject of interest and let your tectonic plates of gray matter do the rest. That's what writing to learn is all about. Don't be fooled into thinking that you have to know exactly what you are going to say before you begin.
Writing is a process of discovery.
Sitting down to write is like priming a pump. It may be very hard at first, but it gets easier with time. Before long it becomes more like throwing a switch or turning on a light instead of pumping a pump handle. The pressure of your experience and prior learning will force the cerebral quakes that we commonly call insight and creativity. Writing to learn and learning to write are part and parcel of the same process.