Hey, my name is qAeG and I main Riven for some time now and I want to tell some of my experimental builds that I've been making up/testing. Feel free to come up with new ideas, i'll try to use as many as I can to give you some insight . Also I will be trying to be as accurate with my guide as possible, keeping in mind that Guides are used by any Elo ( but mostly low Elo).So I will keep it simple . I will just be completely honest with you so you know what to expect of Riven as a new player . Here's what you need to remember:
-Shield scales off AD
-Ultimate deals more damage the less HP your target has (30% HP capped so it's almost an execute
-While your ultimate is active your Q and W range is slightly bigger
-Learn to Autocancel with E and Hydra
-Learn to Wallhop and to Cornercut
-CC is your nightmare
-You're a glass canon ( Lots of damage , lowish HP)

H = Hydra

Main combo (With autocancel)
E + H + Basic + Q + Basic + W + Basic + Q + Basic + Q (Knockup) + Basic
!!! This combo will do huge amounts of damage if you can cancel all your animations properly (Autocancel)

E + Q is a nice longer dash than usual, use the Q + Q + E + Q (Knockup) after positioning well to wall hop or to corner cut, it helps out very much if you can master wallhoping or corner cutting.

E + R + Q + W + Q + Q + R + H (Use your E and R at the same time so you wont be "rooted" in place for a split-second, also try to use the Windslash while you are jumping with Q it will look like you are throwing it while you frontflip and it looks so cool...also you Atocancel the Windslash animation too.

These are the basic combos with Riven that you absolutely need to know to be successful.

Pros
Very mobile
Shield scales with AD
Snowballs very hard
Very good in teamfights and short skirmishes
1/5 potential
Ultimate is an execute
Really satisfying to play

Cons
A glass canon
Autocancel nreliant
Completley useless if behind
Hard combos to learn and master
If you can't master her combos and mechanics to some degree, you will fail...hard!