Welcome. Saw a build quite similar to this on the forums by the user coffeerox.

It's a hybrid sivir build that works quite well when played correctly.

Suggestions welcome!

These runes are based on my playstyle and preferences. If you play better or prefer different runes, use them. Don't follow pre-set builds to the dot.

Attack Speed Reds: Simply needed. You could also use armor pen, I just prefer having attack speed because it fits my playstyle.

Hp/Level Yellows: Survivability, not much explanation needed.

Attack Speed Blues: Gives me 21% Attack Speed, Plus 4% from masteries is 25% attack speed at start. Evens out my build because I focus on AP/AD items.

23/6/1; Once again, just my preference. The added defense is nice as well. I make sure to get 15% spell pen because Boomarang Blade is magic damage. The rest of it is quite obvious, I think.

Ghost: Speed. Always good on dominion. Sivir also can use the mobility and escape, as she has no defense/escape other than her ult and spellshield, which are both on long cool downs.

Flash: The escape is needed. I always play better with Flash/Ghost rather than Exhaust/Ghost, which is what I use to run. Simply what I prefer and play better with. You might play different.

Starting out with a Prospector Blade and Boots of Speed. You can grab health pots with any extra gold you might have, but I personally never remember to use them.

Starting Item Reasoning: Simple. Gives you everything you need to defend BOTTOM. Thats where I play sivir. If top lane is struggling then I might swap with someone on my team if they can handle fighting whoever I'm facing bottom.

~First item is going to be Hextech Gunblade. Makes your boomarang hit quite hard and heal you at the same time. I LOVE the active, and I've gotten countless kills with it.

~Second item will be Guinsoo's Rageblade. You're attack speed will be quick enough to get this charged up very quickly, and I often use a wave of minions to charge it up to 8 stacks before engaging an enemy champion. I use Boomarang blade to keep the stacks up if the enemy champions are far enough away for the stacks to vanish.

~Third item will be Trinity Force. Your boomerang will now hit quite harder because it will give the AD bonus on the skill a 150% boost.

I have not quite tested past the trinity force yet, but theoretical items would be Frozen Mallet for survivability. Typically the games of Dominion won't last long enough for you to get too far past your Trinity Force. You could get the item Entropy, which is built from a B.F. Sword and Phage. Will give you a nice active and plenty of damage, but it's quite expensive and you probably won't finish it.

Either way, after the trinity force I'd personally build a phage to turn into an Entropy for more damage, or a Frozen Mallet for survivability.

I level Boomerang first, as its your major damage skill if you get it to hit twice.

Then I'll level my spell shield second, sometimes putting a few points into ricochet if it suits how the flow of the game is going. (I.E. If I'm in lane pushing minions a lot compared to running around killing people). The cool-down drops dramatically by leveling the spell shield, and at level 5 It'll be around 9.7 seconds, compared to about 16-18 seconds at level 1. Having this sooner in fights can seriously give you an edge.

It's a hybrid Sivir build. Quite simple. You can mix and match items all you want, but I think the core of the build is Berserker Greaves and Hextech Gunblade. You could do a lot of things with this build, and it is def. not limited to what I've outlined here.