Sion can be and will be a difficult champ to play just due to his build(s) and starting statistics. I recommend you to try this build with your friends (or alone) in a custom game, or a Co-op vs. Ai match before jumping into real game. This build can also work for a jungle if you replace any item with the smite of your choice and maxing Q first instead of W.

All of Sion's abilities have to deal with mana so use it wisely and they can be very effective through out the whole game.
-Q has three things to remember: 1. To deal max damage you have to let it "charge" to its full state. 2. It does stun for a certain amount of time so attack or escape as fast as you possibly can. 3. This can not be seen if your in brush on invisible on the map.
-W has three things to remember: 1. It has two effects a passive and the real ability. 2. The active give you more health per kill on enemy champs(this includes assist) and on creeps in lane or in the jungle. 3. The ability gives you a temporary shield that prevents the next incoming damage for a short amount of time, and you can recast the ability to use the shield for damage in a small cluster.
-E also has three things to it: 1. When your roar hits a minion/creep or a enemy champ it slows the target and shreds their Armor for a short amount of time. 2. This roar does AP damage to whoever it hits. 3. This ability (if it is a minion/creep) it knocks them back for the slow and armor shred.
-R has five things to it which makes it devastating: 1. Is when you use your Ult EVERONE can hear it when you use it. 2. As the Ult progresses on the faster you go, harder you hit, BUT it becomes harder to hit as you progress. 3. This can be used for great ganks on any lane BUT be careful when you use it. 4. When you Ult you can activate it again to stop the Ult. 5. When the charge hits the enemy it knocks them up in the air.

-Teleport is used for getting back to lane faster or ganking other lanes.

-Flash is for chasing down enemies or escaping a fight.

-Smite is for jungling this should be a given.

-Ignite is for securing a kill or weaking down a enemy for the assist.

This build was made and used by me in a game as Sion jungle and worked for me; then I tried it out on top and it worked for me even better. This build should give you 25-50% life steal, so you can stay top or get your self back to full health after a won/loss team fight. This build will also make you have more damage rather then the other tanker Sion's.

-Make a major note in your mind that you either kill champions or you get the assist.
-No matter what you MUST get kills/assist and farm to get more stacks on your W's active affect(getting more health).
-Another thing you have to do is farm A LOT to give your self more health/sustain.
-Try to be a lane "Bully" and prevent the enemy to get farm and kills on you and your jungler.
-Sion's doesn't last forever so try not to purposely get yourself killed because the longer you are in it the faster you die.

As you progress as the undead champion who will pound your face in you will become even stronger as him and have more experience as him to help others if they need it. As you play Sion you can mix around with different items to make him more tankyer or have more devastating attacks. Another thing is I just wanted to say sorry for the one blank spot and that I couldn't get pictures of the Summoner's Spells in the build!! I hope you all enjoy this build that I made for you all if so please like and show your friends this build. Thank you.