hi! im 13 as well.so i hope you dont mind if i offer sme CC.
1. are your soldiers fichting it out on a football feild? no. real terrain would have rolling curves. try laminating cardboard and putting plaster overit. also, im pretty shure battlefeilds arnet mowed very often. try adding small tufts of grass with cut off painbrush bristles.
2.also, genaraly whenever thres knocked down brick walls, thre are bricks around them.
3. the od on your us soldiers is wayyy to light. try adding some black.
4. piwashes of dark red work well on figures.
5. your little puddle is the wrong clor. it would be brown on the bottom, because water is clear.
6. nice idea using the stick as a tree. try adding foliage to it.
7.with your exposed wood, weatherit by giving it a light wash of gray and brown.
8. finaly once your done, give it a nice cat of clear flat.