Hi this is JMPzor from Europe Nordic & East. This guide is my first one so feel free to comment what should I edit. I am a Diamond IV player who has played LoL since the start of the Season 2. In my opinion, Ahri is kinda safe pick.

I simply prefer ignite for its burst potential. With flash and ultimate you can avoid getting caught with your 4 dashes. Flash also helps escaping before you get ultimate. Jungler is always the worst nightmare in early game.

Early game try to farm only with basic attacks. Use the mana on poking. But don't get poked while trying to catch a caster minion - just use your Q. It won't be the World's End.

Try to poke with Q when the enemy mid laner is behind its caster minions. Before you get chalice your mana won't be enough to miss your abilities so be sure you'll hit atleast minions before casting. Poking with basic attacks is not bad choise at all.

Bot needs help? On my way for free +1k for the team!
It's often very easy to gank bot lane (level 6+). Try to push your lane and walk down to the bottom lane for gank. The best time for roaming is when you've just forced the opponent to recall.

In team fights try to
1) Help protecting your adc. Your burst will be enought to make their tankiest bruiser under half of its hp.
2) Try to get behind their carries. I usually do this cause enemy team will be very confused if their carry gets bursted down on the backline.

Against ap champion you'll have 35% cooldown reduction without blue buff which means your ultimate's cooldown is around 50 seconds.