So, this is kind of a mixture between medieval and high fantasy, there is some environment work as well as armor concept design.
Well, it would be easier to help you if we had some of your work to look at, but I'll go over what you generally need to create these things. As for the armor, you should research medieval and gothic armor (or whatever you need), you need to look into materials and texture, you don't necessarily need to learn poses, but it would help to understand human anatomy a bit, and keep in mind that the body is round when creating these still frame concepts, you should learn some basic design principles, (such as the 70/30 surface detail, less is better, shape language, color theory). Again, it is difficult to help when I do not know what you have yet to understand.
As for the environments, I can't help you a lot there. Keep in mind atmospheric perspective, general perspective, proportions and textures.