In my opinion, starting off Ahri with a Amplifying tome and building it into Hextech Revolver is the best way to stay in lane and deal maximum damage for low level. but you could also start off with Catalyst the Protector for some durability and regen per level. then i go strait into Will of Ancient for more Spell Vamp and finishing off my boots. It's a Great idea to get Rod of Ages early in game for maximum effect gathering then waiting 10min later in game which could change the outcome. After wards i build into Rylais for more HP and making sure that i can slow my enemies so they wont be able to get away. Finally more AP and Magic Pent.