@yomomma : Well being good with barret depends on pretty much many factors :the way you move , your aim , the way your enemy moves , if you're more adapted to kill while moving or kill while your not moving .
What i would suggest , is just to try to play with it and stick with it . Then with experience , you're gonna get gradually better and better .
But i would suggest if your aim isnt very good to try to stop moving while youre shooting , that way you can lock your cursor onto the enemy making it easier to kill.
But if you're really quick and you have a good aim and you know how to shoot while moving , just try to aim for the enemy's belly , That way if you miss it either gets the feet or the head of the enemy.
If you really want a tutorial or some references , i just googled soldat barret guide but i never used any :
http://elitectf.com/newaccess/index.php?article=363 . I never read it but i just looked briefly and it looks alright if you're a begginer .
Anyways, the best advice i can give you is just to play and play with it youre gonna figure out alot of things and youre gonna get good!
Good luck