When playing riven you need to pick runes depending on the match up or team comp. If you want to scale harder into the late game go 0/18/12 or for early game/laning phase standard 12/18/0.

Runes like masteries are dependent on enemy match up/team comp. Get scaling mr runes for heavy ap team comp or attack speed runes for faster autos.

Riven is very powerful early 1-3, if you know how to play her. If you don't know how to play riven, never go in pre level 3. Try to engage when you are already charging your q and it is on the last charge, this way you can negate a lot of the q's cool down. Never use all your abilities when trading because riven's cd are extremely long if timed incorrectly. Always leave your q/w/e to escape to negate follow up damage. Look for chances to punish laner when they use a ability, that small gap of time is great for you to engage and trade. When level 6 you should all in enemy if they have below half health. Many people underestimate riven's ult, but if they back off don't use all your abilities to chase them.

I personally hate late game. Riven is really hard to play in late game, because she just isn't that good late game, but if you play her correctly you will be able to 1v1 any champion (but the bowser). Your purpose in late game is deal as much damage as possible and run away or die. Try not to engage by yourself because riven gets locked down easily.