when do you use him?
against slow champions (top mid bot)
If your team lacks cc
If you need a mobile tank to protect ad and ap carry
how do you gank?
always to try to sneak from behind and watch out for minions waves because when minions see you the supprice is over. use your crystal slash before you gank so the slow is active when u use it on the champion and use crystalline exoskeleton after you have slowed the enemy to stay on him or use it as a gap closer if sneaking is unpossible, you light wanne buy orakles elixir if you have trouble gank or everything is warded. i recommend warding the enemy jungler close to the spot where you try to gank so you don't get killed by the enemy jungler who is unfortunally at the lane. NEVER dive with skarner before lvl 6 you won't survive...

In teamfight you are the one to engage just use your shield and start the fire use your ult on the most fed carry so some aoe. a good tip for when you should engage, engage if you see that the carry (both ad or ap) is getting away from his team (or if he is not positioned well) so they have a huge disadvantage to support him enagage don't be afraid and use your ult.

in ranked try to do blue and without smite (help from teammate required)and then go straight for enemy red and counter it (if enemy jungle is not starting at red) and gank top for first blood. your claer time is fast enough if you have smite you can do it before the enemy jungler finished wraiths. this is the paths most junglers follow. you can ask top for help. if you did that you will be much more usefull than the enemy jungler. this only works if your blue is on top lane. if other wise the escape paths is to long and have a big chance to be killed.
In ranked it is very important that YOU DONT START FIGHTS WHERE THE ENEMY IS IN THE MAJORITY, always use your head if you engage ( to think obiously not to press buttoms you're not sona....) and try to get an edvantage when your engage ( bad positioned carry, enemy ulties on cd, majority) sometimes it is better to wait for the enemie to engage eg when you have more sustain and shields or cc and the carries in a safe position