well, this is my first mobafire guide i have done so i wont be that good at this.
in the beginning you just want to keep farming up so that you'll have more items in mid game. Riven is more of a lane harasser at about level 4 you want to keep doing your Q, Q, Q, attack, W attack, E and go back to laning phase. At level 6 you would be able to kill the opponent who it is because you probably would have made him low with the harassment. Also you should be able to gank well and gets lots of kills.

I get armor pen reds because armor pen will help you be able to harass harder and allowing your skills to do more damage. I get attack speed yellow because Rivens attack speed isnt that high so in early game all you can do is depend on your combo which has like 9 second cooldowns to do all the damage so you will be able to harass better once again with it and allowing you to get easy kills. I get magic resist blues because you will already have some armor because of that dorans shield in the beginning and also you will get a Wriggle's lattern so armor isnt a big of a deal. I get 1 movement quint because with that you will be able to chase or run away better and allowing you to harass. I get 2 armor pen quints because of the same reason i get armor pen reds.
You can also change the attack speed yellow into armor and the magic resist blues for cool down reduction

I get 21/0/9 because the 21 in offense will help you harass in lane and overcome the lane opponent. Also i get 9 on utility because yes Riven doesnt have any mana but utility will also help you to regen your health. Also i think the exp mastery is necessary for Riven because then you will be able to turn lvl 4 quicker and be able to harass.

I start off with a doran's shield because you want to stay in lane as long as possible and Doran's shield will help you to achieve that goal. Since riven harasses the opponents a lot you will get minion agroed, but with dorans shield with that extra armor, health, and regen you will be harass and maintain the lane without going back. You could also get boots and 3 hp pots if you want. I get berserker greaves most of the time but if you feel like you are getting focused or their is a lot of disablities in the other team get merc threads. Wriggle's lantern is a great item for Riven because it gives the damage for your harassment, lifesteal and 20% proc of doing 500 damage to minions that will help you maintain the lane, and the lifesteal. You could also sell lantern in late game for a GA or any item you wish that will help you win. Brutalizer will let you be able to kill anyone because of that bonus damage and armor pen. you could either get phage next or a blood thirster depending on if you are doing good or not. if you are doing okay get a phage if you are doing super good get bloodthirster. Frozen mallet is good because you will be able to chase better, get hp and damage also since Riven has great mobility skills you will never lose an enemy even if they have flash. Bloodthirsster gives lifesteal and a lot of damage which will help you substain a teamfight. Wits end is gives mr which will help you kill casters with ease and that extra atk speed for extra dmg. You could also get any item you want instead of wits end for a black cleaver. You could also get a GA if you are getting focused too much.

I max out q first because thats the skill that would do the damage when you are harassing. Riven is all about harassing in early game. I max W next because using valor is only when you are running away or chasing people. Its like nidalee's pounce, why would you want to max that out ?
The Skill Sequence is pretty obvious but some people max our valor second, i dont mind.

first time i play riven i got flash and ignite but i barely used ignite so i got exhaust instead and it helped tremendously. You could also get flash and ignite but exhaust is better.

Like i said before Riven is all about harassing in early game, after you have farmed up your brutalizer and lantern you are all set to go gank and get some kills, try to play carefully in teamfights unless you have your frozen mallet, with the frozen mallet your survivability will increase. Try to use your ult in the end of the team fight where it does the most damage, or if someones running away use your ult and hit them.

Pro's
-good harasser
-strong late game
-she's like garen, harass and then ult for the kill
-very good mobility/ will be able to chase and run
- has a stun and knock
- hard to kill with core items
-easy to farm
Cons
-at level 1-2 you have no damage
-get focused
-ult takes some time to use
-needs experience

Try to farm up as much as you can.
Even though you cannot do this is early game you can use your Q, Q, Q, W combo and kill a wave a minions like nothing