You build AD and GA on her, AD because you don't need hp. You're Shield has a very high AD ratio, which means the more damage you build, the tankier you get(One of the many reasons as to why she is OP). GA because once you get going, you WILL and most DEFINITELY WILL be targeted by EVERY OPPONENT WITH CC. If you're cc'ed you're done, that's why many riven players, even if they do kill many other opponents, it takes away XP from teammates, makes your team weaker by comparison, which gives you the pressure to carry them through, which means you're a higher threat to the enemy team, and riven does not do well under CC (This goes for every champion), which means once you die, you're team is screwed.
Let's start out by the laning phase, you should be careful to juggle your abilities, true riven players don't waste their abilities to just dash in, and they mostly only use the Q to dash in, instead of the E. That way you will always win the trade because you'll then be able to use your E to shield their harass.
Don't always go all in on your abilities unless you're sure you can get away safely, always ward to make sure the jungler isn't around, and in under certain circumstances, save a few dashes to get out. So basically use one q to dash in, W to stun, AA once or twice, and E away the return damage that they might offer.
Building CDR on her helps her dish out and take more damage by shielding, dashing, and... Dishing out tons of dmg.
Your ultimate will almost always be up at lvl 16 with 40% cdr. It'll have a 30 second cooldown (Which is quite short IMO).
You'll be able to shield damage many many times in a duel, As well as juggling CC with you're Q(Knockup), and W. Again as to why riven is so op. Much CC, Much shield tankiness, much mobility, AND she has an execution-type of ability, similar to garen & darius(Darius's is not really but w/e). That means the lower HP the target is, the more damage you'll do with your ultimate. Again one of the many reasons as to why she is op.
Again using you're E should mostly be used to block their damage output, E has a relatively large cooldown early game, and using it to dash just wastes it's immense usefulness.
You're able to cast 2 abilities at once using you're E, that way you can save (Alot of) time chasing people down, or running away. The combo is just simply E 'ing first and then the other ability of choice at almost the same time, but E going first.
Usually alot of riven players E -> R so that they don't have to wait the channeling of the ulti to chase someone down and execute them, instead they Dash in and channel the ult at the same time to... save time.
Again most of riven's capabilities rest in on the person controlling her, this goes for all champions. You are not automatically good, but need to understand that sometimes there are global rules (And by global I mean this goes to all champions again) that you need to follow to make your game as successful as it can be.
Vision is extremely important, whether you are gaining it, or denying it. You can cut people off and assassinate them extremely well (Especially with Riven). If you are in lower ELO, you can wait to get you're 6, ward the enemy jungle and wait for their jungler to do a camp, then just jump in on them.
Again some of these include not dieing and knowing when to go in and when to not. Knowing the champion that you play increases the knowledge of how much damage you are able to pull out, to become successful in the fights and coming back out.
Again there are much more in-depth details that aren't even just riven-based, but they are globally out there open to all champions.
Riven just really takes in these global rules of the game and amplifies it extremely well by snowballing if you follow the safety rules, and cheeses.