Shy Combo

Each combo I will go over will have a name and a cretin amount of keys you press to execute in game. The names come from the people who invented the combos. 

The First Combo is called 'The Shy Combo". It is probably one of the most popular combos used because its was the first to be invented. 

What you want to do in game is find a target that is immobile and press:

E-R-Flash-W-AA-H-R2-Q

E is Riven's dash and will allow her to move forward and cast any of her other abilities instantly.

R is Riven's ultimate which makes her sword larger and do more damage. Her ultimate also has a second active when pressed sends a wind-slash that does damage depending on the enemies remaining health.

Flash is a summoner spell that blinks your champion a small distance. You can activate this ability by pressing the F key.

W is Riven's is an A.O.E stun meaning it stuns a small area around Riven.

AA stand for Auto Attack which can be done by right clicking on a unit in the game.

H stands for Hydra. Hydra is a Item you can buy in game that give your champion more damage and an active that does A.O.E damage in a small area. This item is important on Riven because it gives her more damage. The less amount of health a target has the more damage her ultimate will do.

R2 is the second active of Riven's ult. when pressed a wind-slash comes out from her sword and executes targets with low health. 

Q allows Riven to lunge forward and do damage in a small area. When pressed right after R2 she will preform something called dual casting which will cast both abilities at the same time.

When preformed correctly you should have something that looks like this.