Jack, I would suggest doing the following:
Learn a bit of perspective, it helps alot in the long run
Learn a bit of art skills, it goes a long way, believe me (I would suggest Andrew Loomis' Fun With the Pencil:
http://www.gfxartist.com/upload/features/tutorials/7643)
Make sure you know the basics of color theory, this will also help alot
Instead of working on the piece is whole, split it up, separately make the tank, soldiers, etc.
For your people, weapons and such, use reference, I recommend looking at images from Band of Brothers or Call of Duty 2
Look at other pieces of pixel art, look at some games aswell(Metal Slug would be good for you) look at how they're done
Remember: Practice makes perfect, if you dislike a piece, redo it again
Try to not get impatient, sometimes pixel art will take more time than you originally thought
If you want any help from me you can come IM me, I'll be on for about another half-hour and then later tonight(This is assuming you have an instant messenger of some sort).