Hi, im extremeinsanity, a platinum 5 riven main with over 300 Riven games on multiple accounts. I will be sharing the most basic information about Riven in this guide. With this guide you should be able to understand the BASIC things about Riven that everyone that is looking to main her should know.
Current Elo

On riven you mostly max Q, but this depends on the matchup. You almost never max W on Riven, because it chops off alot of your damage in the early and mid game.
If its a matchup where it is hard to trade, you can max E over Q and play it passively. In this kind of matchup, you take your skills in this order:

As for runes, you take 9x Flat attack damage Marks, because this is for your early damage. You take 9x Flat armor Seals, because of the laning phase. You can also switch these with 9x Flat health Seals, if you do not see the use of the armor.
You take 6x Flat cooldown reduction Glyphs and 3x Scaling cooldown reduction Glyphs, to get 10% cooldown reduction late game. Together with Lucidity boots (15% CDR) and brutalizer (10% CDR), and 5% CDR from your masteries, you get 15%+5%+10%+10%=40% CDR.

As for masteries, you will be going 21-9-0 in every matchup as Riven, because Riven is a AD bruiser that needs the 21 points in the offensive tree for the damage and 9 points into the defensive tree for the sustain.

As for items, this heavily depends on the enemy teamcomp. The fact that you build Lucidity boots empowered with Distortion never changes, because it shortens the cooldown on your Summoner spells Teleport and Flash.
The build changes whether or not they have alot of AP or AD damage, or just tanks. Herefor, i will note a few builds and explain when to use them.

Against AD bruisers, such as fiora, zed and talon, you take:
Against tanks, such as malphite, nasus and maokai, you take: You build this, because you will lifesteal more than they can deal. The trinity force is there to help you trade with these tanks.

Against AP bruisers, such as diana, leblanc and fizz, you take:

PROS

-Riven is a very strong snowballer and is great in the lower elos.
-Riven has a very strong shield named Valor that can be used as a gap closer, for a wallhop, or an escape.
-Riven is insanely fun to play, because you can cancel almost every abilities animation. If you cancel these animations correctly, you can deal alot of damage in a short amount of time.
-Riven is a very big threat for the enemy carries.

CONS

-Riven is not easily mastered, and it takes alot of effort to perform excellent animation cancelling.
-Riven is very vulnerable to crowd control, and in the current meta crowd control is a big thing.
-Once Riven loses her lane, she will have struggles coming back into the game, and relies on her teammates for a while.