Just Practice and read up on some basic strategy guides. (I'm only silver so I don't really want to give advice like I know what I'm talking about) but...
Wards!
I'm no Ahri pro, but her spells all count as single target (even though they hit multiple targets) so I would go Rylai's to boost your CC and general survivability. Add in Liandry's and Void Staff to send your Damage through the roof. Deathcap or DFG would be great Big AP items. WotA combos nicely with your passive, and CDR items will do good things with you being so dependant on having your ult up. She's not too prone to mana starvation so some mana regen yellows is probably all that you'll need.
Don't forget to build based on what the enemy team is doing: AP carry is getting fed? You want Abyssal. AD is the biggest threat? Grab that Zhonya's quick! It's ok to end up with some of your core build and some items that are specific to the matchup (not just ok you should really do that). You don't need to build the same items every game, just buy the stats you need as you need them. For Ahri: AP es #1, MPen and survivability are a big deal, and then CDR and Spell Vamp are nice to have.
also, Wards!!!!
And practice in PvP normals. When you are feeling a little more confident go jump back into ranked.
glhf