How to play sniper
Q Shrapnel to slow, push lanes, reveal an area
W Headshot for early last hits and your main source of damage
E To stay out of range of the tower and to fight from the back of your team
R \\\"Secure\\\" Kills

Leveling your skills is depended on the situation. If you want to farm you can max your Headshot and Take Aim. If you want to control the lane it might be better to max shrapnel and use it to keep the enemy from your creeps. Sometimes you can also skip shrapnel all together and get stats. Although at least one point is good for using it as an additional slow or reveal an area.

Positioning is key as sniper and if under attack run away until you lose agro. Try to be aware of the enemies positioning too.
Aim shrapnel carefully for max damage and effect. Learn the timing so you can throw it in the right area. Like sun strike you have to lead your targets.
Careful where you start assasinate you are vulnerable, so try to stay behind everyone, or within the trees.