Boots Of Speed
Health Potion x3
Sorcerers Shoes
Giant's Belt
Amplyfiying Tome
Blasting Wand
Rylai's Crystal Scepter
Amplyfiying Tome
Amplyfiying Tome
Hextech Revolver
Will of Anceint's (Inless you want life steal you can go all the way with the Hextech Gunblade I do not reccomend it because it's kind of a waste with money seeing as Katarina is not any at all Attack Damage.)
Needlessly Large Rod
Blasting Wand
Rabadon's Deathcap
Needlessly Large Rod
Blasting Wand
Zhonya's Ring
Cloth Armour
Chain Vest
Null-Magic Mantle
Guardin Angel
If to squishy at the begining and just want to help out and being targeted get a Zhonya's Ring as soon as possible. That way when they target you. You click it and you turn invisable for a little bit while your team just takes the kills.

My reasoning for the Rylai's Crystal Scepter is to slow them down when shunpo onto them usaly when a player Shunpos onto an enemy they go into there Ulti. There for it will slow them from running away from you and you can give more damage.

Exhust is the same as Rylai's kind of, But if the champion your going against is fed he or she would more likly stay and just fight you while your in ulti there you can use your Exhust it will take his attack damage down and slow him for the easy kill kind of.

Ignite most of you players know they try to get away on low Health and iginite ends there life :P.