Sivir is a pusher, play her like one. Max Q first, W second. When you get E depends on how many casters/disablers the enemy has.
There are many items you can get for her. I prefer Philosopher's Stone, Berserker's Greaves, Bloodthirster, Infinity Edge and Stark's Fervor. You could also get Phantom Dancer, Black Cleaver, etc. Lately I've been toying with Guinsoo's Rageblade as well, but I haven't got a good matchup to test it yet.
Early game keep W disabled, and conserve your mana for Q. It can do a ton of damage if it hits the enemy twice, so try to aim it well. Try to pressure the enemy off the lane, and then take down the tower at level 6 with your ultimate.
Your job for most of the game is just to push down enemy towers. You can help out in team battles with your ultimate, but try not to get into a direct fight.