The same can be said for all theories of quantum gravity, none of them are really making new predictions. I think as a freshman you should not get tunnel vision and only want to study string theory. You should always be open to alternative theories and comparatively judge the merits of each individual theory.
It appears at the moment String theory has lost some of it's steam as there are not as many pubs on string theory and the overall hysteria has died down. But it's still a viable candidate for a theory of quantum gravity. But at the moment, quantum gravity is wide open and an exciting field to enter. And most quantum gravity theories are very mathematical, going beyond the basic mathematics needed for general relativity and quantum field theory. String theory relies heavily on Calabi-Yau manifolds, mirror symmetry and D-branes are another story as well. Some math PhD students are doing their theses on Calabi-Yau manifolds and mirror symmetry, so needless to say, string theory is very mathematical.
For undergrad, I would just take as many physics courses as possible. Some schools like MIT offer a string theory course for undergrads. That's MIT. If possible, try to attend some string theory seminars in your 3rd and 4th years as a physics major and try to see what the field is heading towards and get a feel for what are the open problems (there will be many). Also try to take some advanced math courses, in particular differential and algebraic geometry. I know that is tough to expect for a physics major, but I highly recommend double majoring in math and physics and heavily pursuing geometry and algebra in the math field.
I want to study mathematical physics in a math department. I have the anti and pro string blogs and criticisms but I still think String theory is a viable and legitimate thing for a physics student or a math student to pursue. It's not experimentally verified yet and is far from even having a set of equations, but again, what other theory of quantum gravity does? If you want to work in quantum gravity, you're working in the Wild West, the field is being built as we speak.
Just my .02, best of luck.
I also wanted to throw in, the string theory people have done an amazing job at getting publicity and drawing the attention of many young students. Michio Kaku, Brian Greene, Leonard Susskind among many others have written great popular physics books on string theory and M-theory. I also recommend you read Lee Smolin's books basically criticizing string theory. His "Trouble With Physics" is a very good read and despite many physicists not taking Peter Woit seriously, his scathing review on String Theory in "Not Even Wrong" was very enlightening as well. I really urge you to consider both sides of the argument, no matter what it is.