Your going to start out trying to get last hitting, depending on who your facing, some will try to harass you so you need to try your best to push the lane better then him so that when he hits you your minions will overwhelm him and deal more damage to him when he trys to trade blows. Dodge skill shots, back off and play safe if you have too, you have a ward so use it. wait and call for help from jungle. In almost everygame top lane has a huge advantage over sion and overextends, because if he doesn't sion just free farms. your going to stun with jungle ganks it doesn't matter if you get the kills or not, your first priority is farming to gain health after kills and gold from avarice. depending on who you fight, you can use your stun to stop the enemy from last hitting, I do that all the time, you can even waste your ult when you have like 20-25% health just to fully heal and scare away the enemy. your ult isn't a last resort or 1v1 tool early game like renektons might be.

When you've got some phantom dancer you'll have some damage, you don't ever want to go all out on an enemy, that means if someone like renekton or nasus has his ult and uses it, back out, flash if you have too, he has a huge CD on his ult, and when its down your advantage with your ult and ignite is tough to beat.

During teamfights its really easy and simple to play sion, you don't even need to target the carrys you could easily lock in fight with the tank even, although I suggest trying to stun a carry. sion depends on critical hits to heal himself and his team, damage may look low against a tank, and you may feel you need armor pen, but trust me, with enough attack speed and lifesteal you never die, you just sit there constantly criticaly hitting the tank, healing to full health. If your having trouble doing this that's why you have mercurytreads or quicksilver sash, it stops nasus attack speed slow, ww and malzahars ult, other taunts etc, it also gives you movement speed and attack damage!

When you fight during a teamfight you want to stun get a few hits in and if he runs away like say its an adc or ap you don't need to chase because in most cases they will kite you, look for the closest person and start attacking them instead, because every hit heals you, you want to keep at full health utilizing that lifesteal.