Ash, I'm curious as to what you mean by body language?
The way you conduct and project yourself. You can say whatever you want to someone, but if your body language isn't congruent with what's coming outta your mouth, people will pick up on it being disingenuous.
There's a zillion books on the stuff, I can't think of any off the top of my head, I just read up and observe interactions and even just in movies and teevee. There was a guy called Milton Erickson who was a master at reading body language, I can't recall the book but back in the 90's I was reading stuff by and about him (NLP etc) and some of the stories were fascinating.
Think Alan vs Charlie in 2.5 men, even if the show ran out of funny after a couple seasons, it's a great study in how to act and how not to act. Charlie is always slow, measured movements, master of all he surveys, shoulders back, head up, and a self-satisfied look about him. Alan darts around, flailing limbs, hunched and head down like a naughty dog, never sure what's going on and always worried.
A good tip to help you maintain strong posture if you find yourself flailing and feeling nervous is to whack your thumbs in your back pockets. It will naturally pull your shoulders back, stick your chest out a bit, cause you to speak more from your diaphragm giving better voice projection, and generally make you slow down your delivery - one step closer to badassery.
Someone who's sure about themselves is unguarded and calm, their body language is open, they don't react fast to external stimuli, they have a measured and sure approach, their gaze is strong. The other guy is closed and worried, eyes darting around for signs of danger, the first to jump if something happens like a glass dropped or whatever.
Some guys are natural at this, typically elite athletes etc, they've got that look where they feel like they're ten foot tall and bulletproof, the rest of us need to remind ourselves now and then.