In the early stages of the game, you want to spend most of your time farming up. Use Parley on the minions for the extra gold income, or use it on the enemy to stack your gold passive from the Bandit mastery. If possible, farm with barrels. But in early game, farming with barrels is in most cases only possible when your lane opponent isn't around to stop you. Item spikes will help you farm better.

Your early powerspike is Trinity Force. With this item, you will start getting critical strikes. Trinity force will help you farm easily with Barells, and stack your gold passive very efficiently.

Your midgame powerspike is when you have Trinity Force, Ghostblade, CDR boots and level 13. now you have maximum points in Q and E. You now have capped your CDR and you have barrels that can crit for insane AOE burst. You hopefully have upgraded ultimate too, which means you can more easily push enemies off towers, use it as a zoning tool or just some cross map CC whenever your team is in trouble. Not only can you oneshot minion waves with barrels with these items, but this is when you want to look for teamfights, ONE successful E+Q combo will win you a teamfight.

Once you have your 3 item spike, you can keep your lane constantly shoved by just exploding the waves with a barrel combo, and with sheen proccs you can sometimes splitpush pretty well because you destroy turrets fairly quickly and waveclear better than any other champion in the game.

You can help your team anywhere on the map with your Ultimate, you want to look for places to ult all the time, this will not only help your team win fights or objectives, but it will stack your assists, kills and kill participation. A GREAT tool for a KDA player.

When it comes to grouping up with your team, you want to group when you reach powerspikes, these power spikes are: Trinity Force, 3rd Item Spike, Level 13. Untill then, farm.