There's a couple of things you need to do first; Basics and research. Chibi style, as much as it looks easy - Some look good and some look bad because the difference is that one artist will know basic anatomy where the other will just jump the gun. In order to do well at a style such as chibi/manga you need to know basic anatomy first in order to understand stylisation. So really start understanding shapes in a 3D space, as we are made up of lovely shapes! And start connecting them into a person.
The pose you've gone for is a little difficult for beginners as this pose is about weight and balance. This is where research comes into it. If you hold a rifle... You won't sit flat on your buttocks! Most stand or place weight on their knees. The dominate hand that pulls the trigger needs to have their elbow almost tucked in towards themselves and the trigger is a lot closer to your body than you think it is. A simple search on weapon handling will teach you basic poses. But when holding a weapon, we tend to keep it close as all it will do is smack us right in the face after a shot, your pose looks like its holding a hot potato and I can see the end result being a broken nose!
I've attached a basic female form for you, but keep practising and leaning, you'll get there!
![I just can't draw worth a damn.HELP]()
