The core build give you amazing tanking ability while giving you health and mana sustain throughout the game. You will have a base of 59 hp/5(When Gauntlet of thebes is fully stacked) plus sobek's base hp regen. If the enemy team is denying you from farming, you can switch the Gauntlet of Thebes out for a Silverfox Girdle, which will give you slightly less health and hp/5. Voidstone will give you some well needed magic resist. against mages and Wall of Absolution will give you physical protection. Not to mention that they will both ad to your teams magic damage. The order in which you get the two should be dependent on who you are laning against and what the enemy team is made of. Obviously if you are facing a team of physical carries, you want to get Wall of Absolution first. And if they are a team of mages, voidstone first.
Gem of Isolation III - Gem of Isolation is a good late game item for sobek. It adds to his team fighting presence by adding a slow to his skills, which allready disrupt everyone.
Creeping Curse - Creeping curse is an ability item that should be grabbed whenever you go back to base and have an extra 500 gold.
Divine Ruin III - If the enemy team has a heal comp, then Divine Ruin is a must. It'll reduce their healing abilitiies on top of your Sickening Strike.
Item Not Found - It is a pretty valuable item that will add to your mages damage and not to mention your own while giving you some more sustainability.
I choose to put one point in my first ability(Charge Prey) because if the enemy decides to push early, you might be able to pull them back in to the tower, and if a fight breaks out early, it allows you to keep the enemy away from squishies.
Next I put one point in my second ability(Tail Whip) because if I grab early I can keep the enemies isolated from eachother allowing my lane partner to kill the one closest to him.
Once I get a point in each of those, I max out my third ability(Sickening Strike) because it allows me to sustain in my lane if I grab blue.
Of course I put a point in my ulti(Lurking in the Waters) whenever I can.
You should be laning with your carry or someone with a nice stun. Your main job in the lane is to take advantage of the enemies being close to you. If they push in, punish them by flinging them into the tower and keeping them there by following up with a tail whip. If you push in, fling them back into your creep way and keep them there letting the creeps decimate them. Once you reach level 5 in the lane, use your ulti to push in and keep your mana up.
Mid - Late Game
There isn't much to say about mid and late game. If you are trying to start a team fight, fling squishies into your team so they can focus them down quickly. If your team got initiated on keep your carries safe. Fling them away from your carries, knock them back, do whatever you can to make sure that your carry survives.
Your main role as sobek is to be as annoying as possible in team fights and in lane. Make them want to kill you for being such a nuisance. Be in the front lines taking the damage, and make sure you keep your team alive.