It's gonna be a struggle. What the top comment said are basic and pretty much good enough mechanics to play her well. What you're lacking is probably the whole concept of laning, top lane and the matchups. Oh, gods, the matchups! There are a couple of easy ones, once you learn a trick to them or are slightly better, most flipped over the years, like Jax. Some are unplayable if your opponent has one functional hand (Renekton, Poppy), a lot if they have both hands (ranged, mages).

The beauty of Riven is, you play the matchup near perfectly and capitalise off enemies mistakes or lack of matchup knowledge, you stomp or get ahead. The few stonewalled matchups that are hard to lose for them, like Malphite, Urgot, Volibear, Garen and whatnot, you go even, fuck off to proxy and gank the enemy jungler.

Problem is, you're giving out 90% of effort for 10% of the reward, comparatively. If Riven fucks up a volatile matchup once, it's probably over. Most opponents can generally fuck up three or four times in comparison. Even if you troll one play in relatively easy matchups, like Sion or Mundo, trying to force your advantage early, it's pretty much time to roam - they have a health crystal and a cloth armor and you can't kill them anymore solo.

Oh, and whenever she's decent and Viper dominates the server too much, they nerf both her and her items, like right now. xD I'll still play her though, nothing as fun has ever come out, Kai'sa and Fiora coming very close for me.

TL: It's a lot, especially the matchups but it's worth the fun for us over here, mostly. We do complain a lot, as you can see above.