Riven is a champion that can snowball very very hard but also if you don’t play well, and start the snowball you can fall very back. If that happens get cs and be very flexible on which lane you’re in.
Try to get as many kills you can in team fights. This builds masteries and runes are safe anyways, so if you build and play wisely you will be ahead.

So, we want to make riven snowball, right? If so, we need early game power. So we will be taking Thunderlord’ s decree. The masteries go like this:
Take savagery to get cs a bit easier, then the secret stash to stay in lane longer, than the merciless because it makes no sense since meditation gives mana, after that get the dangerous game for missing health, and then intelligence for the cdr which is very important. Finish it of with the thunderlord’ s decree. When you’ve finished the cunning section go to the ferocity section and start with the sorcery, because you get your damage from your abilities, then the double edged sword since you’re already going to be tanky, then the vampirism for staying in lane even more and finally, if you want to be risky and late game use the bounty hunter, if not get the oppressor which i preferably use.

First of all, the starting items are going to be a long sword and if there is a poke champion get health pots, if not, get a refillable potion, also get a ward. In the first back get boots of speed, a pink ward for vision and I recommend that you place that in the tri-bush near baron. It’s a great spot for pushing if you’re beating in lane, and if you’re in the red side because you can see incoming ganks and you can get away from a gank with your abilities and the boots that you’ve gotten. That means that you should not push when you haven’t got boots. When you’ve gotten a decent amount of gold, go back to base and get caulfield’s warhammer for the cdr and if you can, get the serrated dirk and then finish off the Black Cleaver.

Black Cleaver is a great item on Riven because of the movement speed after aa ing an enemy.
The ad and heath it gives is perfect for a first item. So we’re making a tanky riven that still has a lot of damage. If you are teamfighting often your next item should be the hydra and lonian boots of lucidity. If you 1v1 a lot more I would recommend the yoummus and death’s dance. After those items you should get a defensive item. The dead man’s plate for ad champions, guardian angel for a bit of mixed damage and the sterak’ s gage. Also a good item for ap champions is the maw of malmortius or the spirit visage and for ad champions you can also get the sunfire cape or titanic hydra.