Early Game
You arent him yet, calm down.
Get stun after Lightning Ball if you have a lane partner, if you are solo get Power Surge.
Mid Game
Time to start farming jungle! Your job is make money for the late game, join fights with ult and farm when its on CD.
Buy resistance and healing as needed
Leech is invaluable if they have a lot of antiheal. Unstoppable is required if they are running anything like knockdown or dynamo ult. You can sell Enduring Speed for a bigger vitality item lategame.
Gun seven is cope, don't ever build it he gets so much more damage from spirit and his 3.
Learn where crates and stat urns are, You can get a ton of souls and stats just by running around the map breaking stuff!
Its very important to abuse that you are Seven and also literally him by taking every single jungle camp you can, even enemy ones if they leave them open. Every lane's farm belongs to you. You get the rights to do whatever you want because you are the main character Seven.
You have the biggest and flashiest ult in the game, people *are going to shoot you* so building bullet resist is very important to make you unkillable during your ult, your ult is a DPS check for the enemy team and you want to make sure they don't pass it.
You are the main character.
Advice in build!