How to Ahri:
Charm>Foxfire>Spirit Orb
Use in combination with Spirit Rush for positioning and the extra damage. It's best to hold off on unleashing everything unless you can garantee the kill. If you can do it without the ult, better yet. You can catch them flashing away or evade a gank that way.
Building Ahri...
Doran's Ring is nice base stats, but if you're mana conservative you can do boots first.
Building a revolver first adds beautiful sustain. It just becomes a back and forth with her mana regen and hp vamp. Timing a spirit orb (I don't think that's the actual spell name, whoops) on a whole line of minions (it can be done) regenerates a lot of health when essence thief (her inate?) is charged.
From here I usually follow up with deathcap because of the obvious AP boost, plus the multiplier now added to my additional items. RoA and Rylai's are considerable for the HP and they have high AP ammounts too. Hourglass is considerable for the high AP value and armor. Other items might be situational. I've seen Deathfiregrasp work well, or Morella's Evil Tome for the regeneration and "ignite effect" (greivous wounds?) to slow healing.
As with any champion, do not stop farming...EVER! if you can't get a kill, at least get the minions, all of the minions. Ahri's mobility makes her a very tricky mid to play against too once she's level 6. Learning how to aim your "spirit orb" to just touch the enemy opponent in the turn around is pretty key too for enemies who are overly cautious. You just tap them and they take the magic and true damage and just see a chunk of HP disappear. It's awesome.
This is just a blurb from my experience with her though. I haven't played Ahri in a while.
More importantly, welcome to Mizahar fellow kelvic! I hope you're getting a handle ont he site easily enough.