Sion is a wrecking machine and this build emphasises that. I will keep this guide simple and straight to the point.

Always keep Enrage on, late game you will have a ton of extra hitpoints because of it as well as the extra 65 attack damage.

Try saving your Cryptic Gaze for sure kills more than just the sake of lowering your opponents health, dont rely on it for damage because with this build it doesnt do any.

A fun way to use your Cryptic Gaze is to lure your opponent close enough to your tower and stun them the instant your tower targets them. Using low health to bait an opponent to do this is doable.

Rely on Death's Caress to cushion blows especially if your opponents are range. It can also sometimes mean the difference between a kill or not when your opponent decides to flee and your Cryptic Gaze is on cooldown.
The area of effect on it isnt very large, but sometimes you will find yourself slighty out of reach of a final blow and this move can clench that kill.

Dont save Cannibalism for just champion kills, consider using it to fill up on health when your low and dont want to lose out on experience by having to head back to base.

If you are low on health and your minions are out of your reach with an opponent too close for comfort then you can think about just running into your jungle behind you and using Cannibalism on low level jungle creatures rather than head back to base for some health.

Ravenous Hydra works really well to farm with, so if your anything like me then you can bind the key slot its in to a more preferable hotkey which in my case is my space bar and hit that each time a group of minions come by or even any of your opponents.

Ravenous Hydra also scales incredibly well when used with Cannibalism, you can expect to see your health shoot up instantly which comes in handy in team fights.

These are the absolute basics with Sion and like every champion it takes time and practice to learn to play them confidently.

With this build I'm sure you will have a much easier time getting to grips with what Sion is capable off especially if your able to get an early kill or two.

The items I use here work to build on Sion's Attack Speed, Attack Damage and Critical Strike Chance.

Rely on his Enrage for health, Ninja Tabi and Feel No Pain for defence.

If you acquire all your items before the match is over then sell your Doran's Blade and replace it with:

Last Whisper if your opponents have a lot of armor and you feel you are ok on health.
Mercurial Scimitar if you need more magic resistance and a way to help with the opposing teams debuffs.
Sunfire Aegis if you need more hitpoints and magic resistance, also comes with a damaging aura.

I chose Teleport and Ignite because they suit my play style for top lane.

I like the ability to recall to base for item pick-ups and an instant Teleport back to my lane before my opponent gets a chance to land a hit on my tower as well as prevent losing out on any experience.

Also late game being able to Teleport from one side of the map to the other is something I do a lot especially when their is a build up of enemy minions heading towards a tower.

Works too well for farming and Sion requires a lot of farming.

Being an offensive pusher Ignite tends to help me clench those last strands of health from my opponent for a kill but I also like to use it to scare of my opponents if their harassing me too much.

HOWEVER

If your not confident with playing Sion or soloing top lane, then Flash and Ghost will be better suited for you.

Sion doesn't have any real escape methods other than his Cryptic Gaze to hold an opponent back but when your caught on your enemies side of the map then these two summoner spells will surely come in handy.