He is hilarious, He is great because you can build whatever you want, attack damage/ off tank, full tank, ap/utility (full ap just doesnt make enough sense), and skarnic the speedy scorpion are all great. And you can switch depending on what the game looks like.
At this point I was gonna spend half an hour writing guides for you but here is the jist: play a few games, and then start experimenting, his builds very so much I can possibly write anything in any reasonable amount of time, so get move speed, and then whatever will counter the enemy most effectively.
Skarner is my second or third most played and to be fair I am low elo.
PS try him top, if you win your lane you win top jg cause you can go down there at light speed and either clear it or kill the enemy jg. And if you kill your opponent you have the speed and strength to take at least 11 enemy jg camp before he comes back, which keeps your lane from pushing and lets you hurt the enemy jg