In laning if ur vsing a Bruiser or AD based champ and you feel you will yet be in lane for an extended amount of time, on your first back get 2 Dorans Blades, then go for a Wriggles Lantern. Against AP heavy like Kennen or Swain get an early hexdrinker / vamp sceptre for sustain / 2 dorans. (Dorans are perfect on Riven for early laning, every stat benefits her greatly).

From there if you are snowballing obviously a Bloodthirster is recommended but if you aren't winning your lane or your team needs a tank, go for HP with a Warmogs and rely on your carries to deal the damage later while you sponge damage and disrupt and do good damage with your Ultimate on.

Mid game try to have your Wriggles, Merc/Tabi, and one of your core and building the other; either Bloodthirster or Warmogs. Warmogs beats Frozen Mallet on Riven because you can stack more HP with creep kills and Riven has insane mobility as is, so there is no real need for the Frozen Mallet passive in order to chase and melt enemy carries.

A balanced, dangerous and all purpose late game build for Riven would be Wriggles / Bloodthirster / Warmogs / Atmas / Maw of Malmortius / Boots, and if it gets super late TRADE YOU WRIGGLES for either a Last Whisper, Youmus Ghostblade (remember to activate it) OR a situational defensive item of your choice because by this time your Bloodthrister will be stacked and you will really have no more need for the Wriggles.

Note: I never build Guardian Angel on Riven because you just dont die in well co-ordinated team fights, shes too tanky, unless you get focused hard by their whole team, in which case they're doin' it wrong.

tl;dr riven gud champ, build BT / Warmogs / Maw / Atmas / Merc or Tabi / WRIGGLES (get EARLY but trade it later for a better item [Youmus, Last Whisper, Randuins, Banshees])

go youtube and check out voyboy playing riven ... and you will be successful

If your having trouble early game you can always buy more than one dorans blade, they have the same ratio of ad and lifesteal as a wrigglers but with health instead of armor.

I know it seems troll, but get 3 BT, Boots, GA and Warmogs. Then laugh when you E and get 2/3 your health as shielding. lolol

Pay attention to what you're laning against. Starting cloth + 5 pots/dorans shield will be good against ad champs like garen and renekton, while not so well against something like vlad or kennen.
What skills order you do to get to 3 is preferance, I start with stun, then e, then q but it's kinda whatevs. at 3 you have your full burst combo. E > W > auto auto q auto q auto q auto is the full, you can use q to get out as well. Max W first, E second, Q last.
As for build, Early wriggles is always nice, life steal, armor, extra damage to minions to make farming much easier, free ward. Then what I usually do is frozen mallet and maw for the health, slow, life saving shield, and lots of ad. After this is kinda whatever, as long as it has AD it helps. Bloodthirster, warmogs + atma's, Guardian angel, I usually avoid banshee's for the fact that the mana is wasted but that's not exactly why people buy banshee's. I just don't like wasted stats :3