Weak/Sore upper arms, tremors, somewhat weak legs, stiff neck, tingling in feet and fingers, dizziness for months. Got MRI, clean. I'm 17, what is it?

Weakness, dizziness. Hi Alec: You've had symptoms for several months. Did you have an infection prior to these symptoms starting? I am assuming the MRI was the spine (or brain?). In any case, an evaluation by a neurologist is essential. There are several possibilities, and best to see a specialist like neurology. Good luck!
Labs. sssuming you had labs done like thyroid, ANA, sed rate, B12 to name a few.