Meet Riven
She's a beast. But only in the right hands. The short and simple on Riven's gameplay is this: you need to be very good with timing and you need to be able to weave your skills and auto attacks.(which is really two sides of the same coin) Now another thing to understand, Riven IS NOT a tank. She is actually very squishy. She can't take much damage and in longer fights even a pure AD build taking advantage of her AD based shield wouldn't last more than say three seconds. (the duration of the shield) To counter this I build AS, AD and Vamp. With her high damage giving her more health back per hit, and her shield strength coupled with her little stun, she can drop most enemy champs before they can retaliate.

Also I'm trying to keep this guide brief as it does take a player who is familiar with Riven to rock this build well.

Pros
High burst damage output
Shield skill that scale with AD
Fast and gets faster late game.
Late game gains up to 100 hp per hit and with ult active as much as 250 hp per hit
Fairly good at escaping ganks

Cons
Squishy
Requires actual skill to play

These runes hopefully make sense, if not then you're reading the right section. The marks help with early game ganks and they help tear apart tanks with high armor late game. The seals help her low armor, the glyphs are so she can survive those Ap heavey characters early game, and the Quintessences for over-all survivablity. I honesty can't recommend any other runes - maybe marks of flat AD.

These masteries are basic masteries for damage dealers. 21 offense to increase her damage output that little bit, and 9 in utility just to make certain parts of battle that little bit more comfortable. The masteries I feel are self explanatory. if you don't understand then you should go play easy bots and get a feel for Riven before you go cost your team a game.

So i start with a vamp scepter because its needed for three of the items i build and gives me the option of which i build first, stark's or Wriggles. I get the ionian boots for the cooldown. After those its all about the damage and vamp. Follow the item build and you should be good.

Alternates
Something to look into is the bloodrazor for anti-tank action and/or an executioner's calling for some low cost vamp. But generally I stick to the above build.

First i get ki burst because it has the small stun, highest damage of riven skills, its recharge is reduces with each level, and because I is awesome if you have a good lane mate and aggressive players opposite you.(I've gotten a double first blood with ki burst because they were in the bushes and I stunned them and my lane and I mate tore them up) Next is Valor because its shield combined with the dash allows you to run, or charge an opponent. Finally i get broken wings because its great once your in the fight to weave with her passive, to chase after someone or to run. So theres the initial order, the order i max is ki, valor, ult, then broken wings. the reason is ki has great damage and with a low cooldown you can initiate combat or stun someone and flee. Valor i max next because it also has reduced cooldown at higher ranks and the shield dash combo is a great survival tool. its also nice to be able to dash at someone tear them apart, flee for maybe 2 seconds then turn around and tear up that guy who was chasing you. i've gotten 4 kills in a row like this. Finally broken wings. i max this last because its really only useful for weaving and for movement. the cooldown does not reduce with subsequent ranks and the damage only gets up to about 110 per strike - excluding the AD bonus - which makes the damage kind of an after thought. Her ult you obviously level when you can as it will save you frequently. I say save very deliberately, as it gives an Ad bonus of 20% and it gives you a little more attack range. This will help you obliterate you enemies fast, which is good. whats more is if you feel you're loosing a fight or maybe that master yi got away, you can use her ult a second time - while its still active - to shoot a decent range attack that does damage based on how much life the target has lost.

Now as to how to use Riven. First is understanding her skills. I've explained her basic ones and her ult so now i'm going to briefly explain how to chop people up with them.
We'll examine the skills in order.
First: Broken Wings (q)
This skill is going to be your bread and butter once in the fight. it helps really kick that damage output up. so the idea is to Q > auto attack > Q > AA > Q > AA. this is due to her passive if you haven't guessed(passive explained below). this way you get her broken wings damage and then a huge hit, and it repeats. now something to realize is if you are chasing don't be afraid to drop all three stages of broken wings to catch up. why? because her passive stacks three times. now what that means is not that QQQ = uber AA, what it means is that QQQ = heavey AA three attacks in a row.
Second: Ki burst (w)
This is my favorite skill, because of the high damage and the 1/2 second stun. First realize that at rank 5 the base damage for this is 210. After your AD bonus your hitting upwards of 300 damage at lvel 18. Solid. Now realize that the stun is only half a second. So in that half second you probably want to drop a boosted AA from your ki burst. However, this skill has another more - in my opinion - amusing use. Running. They jump at you, say three of them, you drop Ki and Valor and they're stunned while you skip away happy as can be. And speaking of Valor
Third: Valor (e)
This skill is thankfully at rank five on about a 6 second cooldown. But we'll get to that later first see Valor for it primary use. Movement. Its a dash skill, albeit a short one. First understand that this dash can get you into a fight and out of one. pretty simple. now lets ad on that at level 18 its going to drop about a 300hp shield. soo look at it this way. You charge. they try and kill you but can't because of the shield, but wait the shield dropped, now its getting ugly for you oh look ki burst, they're stunned, and then bam as the shake off the stun you drop valor and just jump behind them. you're still in attack range and ow you've got your shield back. You win.
Fourth: Sword of the Exile (R)
The ultimate. I've already explained it for the most part in the skill sequence segment, but i'll reiterate. I increase attack damage and range for 15 seconds. Basically this is Riven's beast mode switch. The great part about this skill is that it can be recast while the buff is active to launch a ranged attack that deals damage based on how much life the target(s) is missing.
Lastly (and yes this is sort of out of order): Runic Blade (passive)
This is where Riven's magic happens. this is why i kept mentioning skill weaving (i realize that i am just now explaining what this actually does) Riven's passive gives her bonus damage after each skill use. that includes all of her skills, just so you aren't confused. what this does is after each skill use she gets a "charge" that deals and extra 15+ damage on her next attack. (the + is to include the fact that it scales with AD) One thing to note, and something i noted above, is this bonus stacks three times. that DOES NOT mean that you use three skill and hit for 45+ damage. it means you use three skills and then can attack three time with the 15+ bonus. understand? good.

So Riven... She a beast and like any other champ can be played many ways. I am hoping that this has helped you.