I've been having trouble moving my legs lately. More specifically, it the joint in my knee hurts every time I move my leg. Is this just a matter of not stretching propery or could it be something more serious?

I don't really hear much about streching when it comes to talk about cycling. Do most cyclists bother to stretch?

i think stretching everytime is a good thing to do

are you trying to push too big of a gear? have u tried just spinning a lower gear with higher cadence?

my friend quit biking cuz he had a bad knee problem cuz he overdid it and didn't realize it till too late

As mentioned above, try pedalling at a higher cadence. Also could be a fit issue or you may just need to take a day or two off the bike.

Make sure your seat is high enough so that your legs extend fully or real close to fully when you ride. I learned the hard way by not riding high enough my leg fully extended, I was having knee pains (and even cramps once).