Airsoft snipers are a very important part of an airsoft team. They pick off valuable targets and quickly disappear.

[edit] Steps

Buy a good gun. Preferably over 350 fps. Anything under is not going to be considered a real airsoft "sniper" and is a piece of junk.
High grade bullets are essential. .20 is the most common weight. Heavier bullets like .25, .28, & .30 are used in more powerful rifles. Use .20 with 300-350fps. Use .25-.30 with 400fps and over.
If the gun you have received is at least a bit good, then it will have an adjustable hop-up. Use this wisely. Do not turn your hop-up to its max with .20 bullets using a gun with 400fps. They will curve horribly. Buy at least .25 to .28 and turn your hop-up a pretty good ways up if not all of the way and the bullet will fly straight and fast.
The length and "tightness" (bore) of the barrel play a large part as well. Longer barrels mean more consistent and higher fps shots. the diameter of the inner bore of the barrel also has an effect upon the consistency and fps of shots. Also, consider purchasing a gun that shoots at consistent speeds. A gun that pumps out random speeds from 350-450 fps for every shot will not allow for you to make good shots.

[edit] Tips

Do not be fooled by the looks of a gun. Read reviews and make a wise decision with a buy.
Less than $50.00 for a sniper rifle is not usually a good buy, as it can probably be outrangedby a high-end AEG. This makes the sniper pointless. Buy a sniper that can significantly outrange your opponents, giving you an edge.
The UTG L96 Shadow Ops Sniper is around $180 and shoots 480 fps consistently, it is a cheaper spring option to the expensive snipers like the M700 gas sniper.
Wear eye-protection specifically made for airsoft.(Polycarbonate lenses)
Wear a vest that has body armour. It's not fun getting shot in the chest by a 400fps weapon. You can get airsoft products at Airsoft Sniper Rifles