I guess many people knows how to play so one thing remember about that you are an ***asyn you have to kill quickly and vanish.

During fights 1v1 First take as much HP as you can with rake (W) and on the higher levels jump on the enemy (E) when you have all CDs after first hit use Noxian diplomacy (Q).
In the case of runnuing after someone try not to use Ulti but jump on mobs (Q) and slow down with rake(W) if you cant do this speed up with "Yuumu blade" or ulti end then jump (Q) into the enemy.

Try to avoid direct fights becouse you don't have so much armor or hp so try to stay in the shadow and when enemy team have below 50% hp (Except tanks) jump into them and try to slow them down then cast ulti.

I don't remember on what lvl you can take what skill but try to max Rake (W) first, always have 1lvl of Cutthroat (E), and try to max Noxian Diplomacy (Q) and obviously take ulti as soon as you can.