If you decide to play on the highest difficulty level, the gameplay will be strongly affected by the distance from the target and the wind. It's worth to remember that when you're at a higher place than your enemy, the correction will be smaller and higher in the opposite situation, when your enemy is inside a tower or another building.
There are three pieces of importation above the scope - the distance, wind strength and the current zoom level. The wind direction is determined by the white gauge below the information. If the gauge is filled to the left of the centre - you have to pull the crosshair to the right. If, on the other hand, the gauge fills the right side - place the crosshair to the left of your victim. It's worth mentioning that the correction should be lower on smaller distance.
There aren't many different sniper rifles in the game, though each has a slightly different scope. With time, you should learn to determine distance based on the size of enemies in comparison with the scales and current zoom.