Well there's a few things. Make sure you are good at math. If you've done well in Trig/Algebra 2/PreCalc you're good for Physics B. Then, if you want to do Physics C, know calculus.
Plus, most people take Physics C as a 2nd year course... so it's not STRONGLy discouraged, but discouraged all the same since you depend on Physics B for some stuff.