Riven is an amazing early laner. Her level 2 damage is insane, as is her mobility. Take as much advantage as you can of how strong she is early, because if you don't, your mid game will lack. Her passive makes her damage output so high that she's near impossible to deal with. That on top of her shield makes her a formidable lane opponent. Riven can clear waves really quickly, and also ignore them completely and jump straight to champions. All in all, her early game is fantastic

Riven's mid game WILL be game changing, either positive or negative, that much is up to your early game. If you win early, start farming behind the enemies tower and try to force him to fight you for creeps. Whenever he goes back, be sure to either harass the enemy jungler, or gank mid. Try to keep the tower alive for as long as you can, you don't want to kill it to early and let the enemy farm back into the game. Everytime your ult is up you should be looking to fight. Alternatively, if you did bad early, then hug tower and only farm when it is safe to do so.

Riven's is really disruptive late, while at the same time dishing out tons of damage. Her ult on top of all her other moves provide an insane amount of damage dealt to the entire enemy team. Your main goal should be getting to the enemy AP or AD carry, whichever is easier to get to really. Try to use your stun to disable their peelers for long enough for you to hit the ADC or APC for a good chunk of damage. Don't be afraid to die. It's worth it if their entire team focuses you and leaves your ADC alone to pick them all off. Half of your goal SHOULD be to direct the enemies attention on you rather than your squishies. Remember to ult before you do any of your combo to maximize your damage. Over all, riven is a solid laner that can turn a good early game, into GG for the enemy team.