EARLY-GAME:
Farm safe against your laner. If it's someone, who has a stronger early-game than you or outranges you, try to move in for creeps and otherwise stay out of his range, just to get the XP you need to level-up. If you realize YOU can pressure the enemy and no enemy jungler is near and midlane isn't roaming, go for it. You can use your W to gain range against someone like Lucian or Graves too, so you can farm safely, since it's range is pretty high.

MID-GAME:
Don't focus TOO much just on farming anymore. Look for global map pressure you can put out with your long-range E's and Ults. If you see any opportunity to engage an enemy when you're ahead or catch him off guard, just go for it. Your Ult is a great tool you can engage with out of nearly every situation.

LATE-GAME:
Farm to push nearly only now. You should also try to save your ult for disengaging now, not just Yolo-engage into the enemy team. Especially if you're playing against some spooky-*** assassins like Katarina, Rengar, Talon or especially Kha'Zix! Still, if you see opportunities to engage and carry a fight, use whatever is necessary.