I have just started recently developing my drawing skills.
All I can say is; detail!
For your first few drawings, just pick a picture of a spartan, then just draw lines first, every line you see in the spartan, draw over it a with 1 pixel black round brush.
After that, colour the whole thing yourself, part by part.
First the helmet, then the shoulders, then the chest, so on.
First take a medium color for (for example) the shoulder, then slowly build up to the light and dark parts.
I painted This spartan myself 2 days ago, took me about 9 hours (using a mouse).
If you look at the leg, I took a middlecolor for the green, and painted the whole leg, then I zoomed in around 700% and started adding other tones of light and dark green, with a 1 pixel round brush.
When you've practised enough you can start creating your own spartan.
Also, if you want your drawings to reach the multiplayer BETA screen, you will need years of practise, and a creative mind.
If you really love drawing, spend a lot of time on it, practise makes perfect.
[Edited on 01.18.2011 7:53 AM PST]