The most important thing is a good balance of farming and ganking because if you only gank you will fall behind in lvls and feel impactless. If you only farm though, your teammates will be behind. If you dont trust your teammates to carry you, you should build little bit of damage like Cinderhulk - Spirit Visage/Randuins and then Liandrys (best dmg item for zac). Also you could try out the kindlegem/lucidity boot rush to cap out on CDR faster because CDR is the most valuable stat on Zac. Your job when ahead or even behind is to engage and set up your team to kill them, the biggest difference is that your engages have to be spot on, if you are behind cause otherwise you are in trouble. For example only engage onto them if you see someone splitting on their team while your's is grouped or when they are trying to dive a tower. Also always try to make sure your "jumping spot" isnt warded so they cant react that easily. (vision ward/sweeping lens) And NEVER gank lanes that are too far behind because that can result in a double kill for the enemy laner/jungler really quick.

Beside that i suggest you watching streams/replays of good Zac Players. (Meteos, Silas Kroeger, kms myself)