I'm not a Riven expert, but the amount of hours I've spent watching Dekar173 and BoxBox is enough to give you these basic tips. I recommend watching these two if you want to learn about her in depth.
Riven, despite what anyone may tell you, is an extremely mechanical champion with both a high skill-ceiling and a high skill-floor, the latter being the most important part. It's going to take a lot of practice to be able to play Riven at the same tier that most people can play champions like Renekton or J4, which is why a lot of people never touch her (like me).
The first thing you need to learn as Riven, however, is managing your cooldowns. You're a relatively fragile champion who relies on mobility and a 1 second shield to keep you alive. This means that you're extremely vulnerable when your spells are on cooldown, and thus need to get associated with the amount of time it takes for every spell to become available again. Once you've got this down, you'll be able to exploit her strengths like winning Top Lane trades by using your long combos, backing off or absorbing damage with your shield, and re-engaging when you're about to have your spells available again.
The biggest thing that separates a good Riven from a great Riven is animation canceling. Before you can even think about playing Riven at a high level, you need to spend a significant amount of time in the practice tool working on things like "The Fast Combo" and combining your damage abilities with your Shield. Look up all videos about how to properly accomplish these, and spam Adrian with "how do I shy combo" donation messages, then perfect them in the practice tool and norms until you're ready to take them into ranked.
Finally, you need to understand your role as Riven. You're a good combination of a Skirmisher, Diver, and Assassin all packed into one, with a unique mix of each class's strengths and weaknesses. You win most 1v1 fights, and are also great at 1v2s and possibly 1v3s, but only if you're snowballing. You have high mobility, meaning you can chase and escape with relative ease, but you also are completely useless if you're locked down. You have a healthy amount of CC and AoE burst damage, making it easy to set up team fights, but you're usually too squishy to survive if you're the only engager on you're team.
So where does that leave you? Well, you have different jobs depending on the state of the game, with variables including team compositions, early/mid/late game, ahead vs behind, and who's individually fed on each team. If you're the only fed person on your team, you could split-push if your team is able to hold off the enemy, or team-fight if you're confident in bursting down the squishy backline. If you're behind, you should build more defensive items and work on holding the frontline or suicide diving the backline for your fed teammates to follow up with more damage. It's up for you to make the right decisions based on how the game is going. Look up guides on how to play as a diver, assassin, and skirmisher.
This the best I can give you from my limited experience with the champion. Sorry it's so much text :P. I hope it was helpful.