So I think you mostly just need to brush up on proper anatomy. I get the feeling you've either learned largely from looking at other drawings or you just haven't spent enough time drawing realism. In addition to gestures and life drawing, I'd suggest taking the time to draw skeletons, muscles on top of them, then the skin on top of that. That sort of break down can go a long way in teaching you how things work.
The reason your pic here looks "stiff" because the pose isn't very natural for sitting and the weight distribution is off. You kind of have an in between going here--to look natural, the person should either be leaning back or forward more. Here's the example I drew:
Personally I think 1 looks best. A person's center of gravity is at their core (hip area, about) so in order to balance, their weight has to be distributed across whatever limbs they're using.
The other thing I noticed is that the armor doesn't... look like armor? You have lines that kind of insinuate it's supposed to be armor, but there's no structure to it. It almost looks like they're wearing a shirt with lines printed on it. D: To fix this, you have to make it look a little more rigid and give the plates proper space on top of each other, like this:

Rather than JUST thinking about the design, also try to imagine how it wraps around the body and even how it's constructed or held together. It'll go a long way in making it look more believable! That goes for regular clothes, too. 