Riven is one of the Greatest duelist in this game. If right played she can do anything, even get double kill then ganked at level 3 or 4. She's not super tanky, she get her tankynes from e (shield), decent lifesteal then has decent amount of damage.

Pros:

high burst damage.
high mobility.
good escape.
can kite others.
can carry game if whole team is feeding.

Cons:

Can be easyli bursted down.

Dashing far range: e >> q >> aa >> w >> q >> aa >> q >> aa >> aa >> aa...
Dashing short range: q >> w >> q >> aa >> q >> aa >> e >> aa >> aa >> aa...
Dashing far range(if tries to run then dashe): e >> q >> w >> q >> aa >> q >> aa >> aa >> aa ...
initiate with ulty: r >> e >> q >> aa >> w >> q >> aa >> q >> aa >> r >> aa ...

Actually there's a lot of combinations, it depends on situation for eg.( if run's to use stun, or if stays to empty pasive stacs and save closer gaps). AND NEVER USE ULTY IF ENEMY IS HIGHER THAN 50% health, most damage is dealt with ulty lower than 30% health. Ulty can be saved until enemy tries to flash away.

MOST IMPORTANT FACT ABOUT RIVEN HIGH DAMAGE BURST. Riven aa animation is over dramatic, and slow. Damage is dealt at the beginning of the animation. And after broken wings ability, he imiadiantly does another aa. For eg (riven uses q, then aa, dont wait until animation ends, just then you see damage was dealt press another q!!!!If you practice it to do it as fast as lightening riven burst damage will OUTSTANDINGLY HIGH!!)

Why to use another q after w. Notice one thing, after using w another aa will deal damage about after 1 second or more.... we dont need that, we want damage as fast as posible, so use q that aa will be used as fast as posible.

To get insta ulty ready. After preesing r use instantly e or q, and your ulty will be ready in no time. Same thing with over dramatic animations stuff...

Try using smart cast's, gives you ability to press skills faster.

9x atack damage marks better than armour pen runes, because hes q does more damage, that can be pressed 3 TIMES !!, more damage to w, and gives more shield for e !!
9x armour seals, top lanning, mosttly going against ad champ, specialy garen (hes bigest couter), and it gives defense against minions and turrets, more importanly against golems
9x cdr runes better than magic resist, because riven needs cdr, and early cdr for riven is OP, even if going top against ap.
2x life steal quin, 4% of lifesteal is huge save for riven first blood, and regeneration of heatlh.
1x damage quin, to gain more damage for all riven abilieties.

Why offensive tree is best for riven? most importanly for last point that (increase damage by 5% bellow 50% health targget) this point combined with riven ulty is sick, riven ulty damage get's 5 more %..Putted all masteries to maximazi damge output, putted 4 point on cdr, because as mentioned early cdr is op for him. Riven doesnt need critical.. Why put 1 point better on tough skin - cause of going against golems, if you freeze your ane a lot, then ataking enemy if you and your enemy has a lot of minions, you will get major advantage, eg (like about 15 minions on each side, all of them at that fight may attack you 4 or 5 times, so that means you will get 15x5 = 75 damage less from them, it counts as you will get about ~100 hp more than your enemy top lane, or jungle, at early game huge advantage)

strarting items elexir and 3 potions..
elexir - to give you a heal that isnt affected by ignite, more ad damage, for nice amount of time, and more health .. eg(your fighting for fb, you both are at low health use potion, even if ur ignited you get good heal, wich will be ur advantage in fb kill, and will let you live even if you got ignited, after that you may b, having around 1000gold, buy 2x dorans ward, and pots to have 3, than you come back to lane potion will still be working giving you about +300~ more hp with dorans, + ~30 ad damage, and youll get on atack 10 lifesteal ++ to be more survivable) after that you will crush top lane, and can easyli snowball afterwards...

Wards and potions always remember to leave gold for them, they let you stay ALIVE !!

if afterwards your getting snowballed so more and you get back after that around with 1,7k gold buy bf and ward, if you got less, buy vampiric scepter, afterwards buy bt firs

one more pointer, why buy boots, if you got plenty of mobility and there is a greater item?!??!?

last wishper it is a core item after bt cause of armour pen, and 40ad damage, it is cheap and can be fast bought around 16 min or earlyer,

afterwards, mostly everygame buy black cleaver, because it has nice cdr health and dc damage, + nice armour pen.

other posibilities:
hydra - if enemy team ahs a lot of thormail's, double lifesteal can counter it easyly, and if you need huge aoe dmg in tf.
Zephyr - if you are beeing kitted hard best way to counter it, of its movement spead , tenacy pasive, and cdr to use skills more faster, to run or caught someone,
Guardian angel - good way to counter if both ad and ap is fed. and if getting focused a lot(still after getting it you need to avoid beiing focused).
Radium's omen - this item is great going against fed ad players, cathing players, kitting enemy, decent survivabilty from health.
Mav - that pasive shield with e works miracles against ap fed enemy players, you'll counter them easyly.

If you are not warded try to not push and only last hit, and keep minion wave around your tower, but dont let tower hit enemy minions, freeze lane if posible. On other hand if you are warded, push minion waves to enemies tower, then you have pushed go into jungle for golems, then return, push back lane to turret and go for volves. if you pushed minion wave and neither volves or golems are up, recall, or either go roaming, your decision.

A quick straight guide, for core facts about riven, to get you into handle of him, it wasnt created to look beautyfull, just info about Riven, how to win every top lane game.... And sorry for my grammar mistakes, and everything else that is wrong in this guide, my first guide, hope you like it. :)