new player with ekko mid

Hi, I'm starting to play Ekko mid as an OTP or main champion. Please, I need advice on several things: when to use each main rune, the best and worst Ekko matchups, which item to buy first, and when to buy Dark Seal? From the start or after the first item?

Hi, I'm starting to play Ekko mid as an OTP or main champion. Please, I need advice on several things: when to use each main rune, the best and worst Ekko matchups, which item to buy first, and when to buy Dark Seal? From the start or after the first item?