That's still pretty early to start thinking about nonlinear systems. Just learn as much as you can from your current courses and grab a differential equations book for the summer to self study. Any differential equations course or book will touch on nonlinear systems after you have a feeling of regular DE's. You're going to need some basic linear algebra to start also for the Jacobians. Analysis can't hurt either. I would highly recommend learning MATLAB right now as well. For me, nonlinear systems are best understood qualitatively because the math can get kinda nasty. Punching a non linear system through into a plot explains a whole lot.