Ok so here is a sivir build i think is good.
Please dont blindly follow this its merely suggestion of items i think that will work well with her. buy these in order of necessity what you need at the time.

i left the 6th item of to your decision heres some suggestions

if your against a team with alot of health get a Madred's Bloodrazors

if they have heavy armor get a Black cleaver

if they have magic resist get a Wit's End

if they are heavy in ap damamage get a Banshee's Veil

if you need armor get a Guardian's angel

if they have a heavy auto attacker get a Thornmail

if your doing just fine and wanna amp your damage output some more get another Bloodthister or Phantom dancer or Last Whisper or Atma's Impaler or any of the first 3 suggested items