Hello people,
this is my first "League of Legends", guide. I'll explain why I'm using the build, that I'm using. It has more potencial in it, then you probably think after the first view. Enjoy :D

I'm using armor penetration marks to build up for the mid/late game. I don't recommend ad marks like a lot of people do, because the armor penetration marks are probably more effective in the mid/late game. As I experienced, you'll get out better with them. I've seen a lot of Riven's or Xin Zhao's going with full AD runes and they were really strong with them, but they'll get weak in the late game. I'm using flat armor seals for the early sustain and magic resist per level glyphs for the mid/late game. At least I'm using AD Quints to raise my early damage.

I'm going with a 21/9/0 mastery's page. I'm putting 21 points into "Attack", for the strong damage and then I'm putting "9" into defense, just for a little bit more early sustain, because I'm building first AD. Try it out, it works fine for me. I don't recommend to not put 21 into Attack, because your damage will be very weak as a Riven. Better don't do that.

I've choosen this items because

1. You'll have a lot of HP and a good amount of armor, while your AD is still very high.
2. You'll have a very good crit chance and strong critical damage.
3. You can at least build more magic resist or more penetration, whatever you need.

Don't forget to buy a second "Doran's Blade" for the early sustain, its really good.

First I'm bringing my "E" to level 3 for more early sustain. After this, I'm bringing my "W" to level 3, the additional damage and sustain will make you really strong. After this, I'm maxing my "E" for even more sustain, because more sustain will help you a lot in killing the enemys or in a "1 vs 2 gank". I'm letting my "Q" at level 1 and I'm maxing it at least, because it doesn't bring you a lot, you aren't team fighting much in the early/mid game.

When you're stronger and you're playing against an weak laner, you can just jump in with your "Q", then stun and then auto attack the enemy. At least you have to dash away with your "E" to tank some damage or you can dash in first, while you're enganging. When you are having trouble with the enemy laner, jump in with your "Q", stun him and then dash out, do the same for getting some last hits, if he's trying to haras you. Finish him off when he is weak enough ;) Otherwise after you are having your ult, it should be anyway hard to kill you in a 1 vs 1 :D

As Riven you have to wait for a good moment. Your timing must be perfect. After the other team started an engagement, you have to pop out your ult, jump in with your "Q", then you have to stun them, after this use your "E" and start auto attacking. Don't forget to pop out your ult before its on cd!