Buy this textbook & read it
also a useful quick reference : http://www.dspguide.com/
DSP is really useful in a wide variety of disciplines, especially if you end up becoming an embedded programmer. A lot of embedded code hooks into digital or analog sensors, and gleaning useful information from these sensors usually requires some amount of signal processing. Granted, it isn't always as complicated as what you'd learn in a DSP class, but it is really useful to know that stuff is available when you need it.
All this being said, I'm not going to lie to you, this material is not exactly intuitive, and can be very challenging to learn on your own. It is very math heavy, and there aren't very many good resources out there that I know of that can help with the learning curve.
Things that I used to help me get through the classes were:
I'm sorry to hear that the main prof is shitty. Maybe another thing to consider would be taking the class online from another university? This one looks pretty solid.