Use machina/piss rifle/huntsman and go for bodyshots only :D.
But in all seriousness, a good sniper always has a good position. Make sure that you are positioned in a way that will have your team for support, in enemy line of sight, and as protected as possible. Also, utilize secondaries and melee's. If you prefer more cqc, I'd recommen taking either the smg or jarate (with bushwaka). I usually use kukri, but the shiv is pretty good. Since I'm not the greatest shot, I can rely more on game sense and utilities to make me a bigger threat then I would be. And that's the key to sniper, terrify the enemy.