Rushing Will of the Ancients then Rylai's Crystal Scepter will allow for ALOT of chasing and sustain early and late game. By the time you have Rylai's you should be able to burst down most of their team including the tank. Getting Deathcap pretty much garuantees the kills you're going for. Abyssal lowers their MR and Increases yours so it's a great item to get if you go into a team fight or just ganking them one by one using your ult and your fox-fires.

Pros:
Kills anyone,
Great for Harassing hard,
Great for getting in and out without a scratch,
Easy multi-kills,
Carries you and your team.

Cons:
If you don't use her R-W-E-Q-R-R-W Combo correctly they might live,
Your still squishy so you can't take them all on (Unless you know how to harass with Ahri very well.)
and Silence will shut you down if you don't get to the silencer first.

Playing in Normal and Ranked will be the same with Ahri. So I suggest you practice her combos on Normal drafts before you go do some Ranked games. Once you completely dominate a Normal game pretty much by yourself and they surrender because they can't step 2 ft. out of their pool because your at their doorstep waiting for another penta. That's when you know Ranked games will be a breeze. 1200 - 1400 ELO.