Not Updated For Current Season
Early game: Play defensively. don't expect kills. Make sure you have enough for boots and lifesteal when you bp the first time.
Mid game: Get golem, then either jungle more or go to a lane that needs help. In Team fights bring up shield first, then stun whoever needs stunning, then switch to phoenix stance and hit the nearest squishy. rinse and repeat. you can afford to be a little reckless if cleanse/flash is up but you are still pretty squishy yourself at this stage of the game.
Mid-late: Once you have malady and veil you become very mobile and durable. You should have enough attack speed for your phoenix stance to be a threat to the other team's squishy.
Your role in team fights is to be the fly that the other team can't seem to swat away. You should usually jump in right after the initiator and get the other team's attention with phoenix stance. Your veil should make you tanky enough to endure the entire fight without fleeing. However, if the entire team starts focusing you, bear stance + summoner spells allows you to get away easily. The people who are determined to kill you will waste their time chasing you because bear and turtle will make you very hard to kill. If the other team chooses to ignore you, then you can cut down their squishy with relative ease.
Late game: Once you get a good damage item, you can backdoor towers easily, or be even more of a threat in team fights.