to do more u can use select on Xbox or Back on PS3 to spot enemies for your teammates...Eventually when unlocking the mav or soflam u can use those items to easily spot enemies(MAV) or laser designate targets(Soflam) to help your teammates take them out

For sniping, I'd recommend not shooting until you have a sure-shot on your target.

No body shot is "One Hit, One Kill", so if using a Bolt Action rifle, achieving a head-shot is the best plan. If you only get a body-shot you may be have difficulty pulling back the bolt and re-aiming before they flee / hide.

If using an Semi-Automatic Rifle, try and double tap the enemy to get a fast kill, while also trying to aim for the head / upper body for best hit chance.

Also, this isn't Call of Duty, Snipe from a good range, snipers' cannot hold ground against assault / support classes at close range.

You could start by playing assault/support/engineer