Riven is an extreme feast or famine character, so if you pair her against a passive laner like Shen or one on the squisher side like Teemo you'll do well most of the time if you play aggressively.
Best advice I can give is to use her passive. Weave an auto attack in between her abilities to squeeze out every bit of damage she can. if you use an ability after she has 3 stacks you're wasting a lot of potential.
Another tip I can give is to abuse her Q's lvl 1 presence. By that I mean once the first wave is about dead (because minions hurt a lot early) see of you can pounce on them. If you stagger autos in between most champs will lose abut half their health. Squishier champs may even die of you follow up with an ignite. Be careful though, gauge your opponent's confidence, if they're a dueling champ like Darius or Fiora they'll just turn back against you.
Lastly, for a smoother engage try doing Q1 (first Q) --> E --> Q2. Q has a very tiny internal cooldown, so if you wedge your E dash in between you cover the same distance a fraction faster. Doesn't sound like much but it makes a difference sometimes.