With most mid lane Assassins (Ahri, Kassa etc), you'll find that especially during low level of ranked games, you really need to be able to carry. That means you need kills. Kassa, Ahri especially are great at roaming and you should be ganking most lanes at every available opportunity and warding so you know where the enemy jungler is.
Minion farm for middle isn't _as_ important as say ADC or top (although still important), but providing that you roam and kill lots you can subsidies going behind a few minion kills, as the gold you'll get for killing the bot lane and jungler should subsidise.
With Ahri as I'm sure you've worked out, she's all about burst damage, spirit rush into the enemy, charm, then Q, W, auto attacks and the spirit rush to get outta there - providing you have good AP damage, most champions will fall to that combination - especially after a few kills early on due to roaming.
With regards to wards, ward next to the enemy wraiths to pick up ganking from there, if bottom are warding properly you should see the majority of ganking from bottom attempts (and also pick the enemy jungler up) and same goes for Baron pit. I like to stick a pink ward in at the red buff / blue buff, so I know exactly where the jungler is and can plan on taking him out. Generally the jungler just roams around on specific patterns looking to gank most lanes - these are easy to pick up after failed ganks they'll still just roam around looking to life steal rather than going back. Lay traps for the jungle and you should pick them up no troubles
