Sivir was really good back in season three. Remember? There seemed to be one of those evil Boomerang blades everywhere. Well, Riot noticed this and decided to fix it. They took her mana. Like all of it. When The http://www.mobafire.com/images/item/essence-reaver.gif Essence Reaver popped up it seemed to get alot of hate, and it still does. Honestly though, it's the answer to an AD carry's prayers, espically a Sivir main's. Sivir has many good things about her and a few Drawbacks, so witout further adieu, let's dive in.
Sivir's Pros

Extreme poke with her q
Her W makes her an

insane lane bully
Scales very well with items
Has a spell sheild that makes her almost flippin' invinceable to Crowd Control
Auto attack speed buff
Fantastic wake clear and push
Ult that can give the entire team a speed boost

Sivir's Cons

Very short Auto Attack range for an AD Carry
Very mana hungry until you get your http://www.mobafire.com/images/item/essence-reaver.gif Essence Reaver
Fairly long cooldown on her Q early, a long cool down on her E the whole game
AA speed buff doesn't show up until you get ult at 6
No true escapes besides her ult, which you don't want to use as an escape.
Her E can make it hard to Last Hit if not used correctly
Susceptible to ganks due to her natural pushing power (All you have to do is ward)

That about does it. These are most of the things you need to know about Sivir to be successful in lane.

Not as straight foward as most builds I know. Most of them want you to Max that Q for maximum poke. Well, that can come in handy, but what Sivir truly excels at is bullying. Now, bullying means making the enemy AD Carry decide if they want to risk taking damage to get farm, or if they want to lose farm and keep they're health for a trade or kill. In either scenario the bully usually comes out on top because they either have

1. More farm, and therefore more gold.
2. More health, and therefore the ability to win trades and earn gold more easily
3. More health, and more Farm.

So, taking W first, Getting two in the Q early and maxing the W as quickly as posible will allow you to, in almost every circumstance, bully the enemy AD Carry.

It main seem strange to many people, but the first full Item you should build on Sivir (other than the boots I mean) is the http://www.mobafire.com/images/item/statikk-shiv.gif Statikk Shiv. This, paired with the two dorans and B.F. Sword you should have gives you godlike Lane push. Like I talked about earlier in the skill chapter, this adds to Sivir's Bullying capabilities.
Next, you want to get the http://www.mobafire.com/images/item/essence-reaver.gif Essence Reaver. This Item is Essential to Sivir's kit. The truth is, Sivir needs mana. What better way to recieve mana then doing what AD Carry's are meant for? Dealing damage! With this item you can get mana from Auto Attacks!
Following the Reaver you're going to need Crit Damage, tons of it. What better than the Infinity edge? 250% damage crits? Heck Yes!
The Final core Item is the Phantom Dancer. Not much to be said...just minions, STAY OUTA MY WAY!

In short Sivir is a bully. Play her like one. Be mean. Relly mean. But don't get to agressive. Remember to ward your river, use the ult when you see the chance to engage. And of course, HAVE FUN!

Hope the guide helped, got kick some butt!