Singed is one of the best farmers, an amazing tank and the best teamfight iniciator. With this build, you will become a lifesaver of your team, you will be catching the running enemies, since you are the fastest hero on the map. When master Yi realizes you run after him, he wastes his ulti, just to get out of you.
This build is not focusing on singed with 6000 lives, 5000 mana and a lot of ap. That build is usseless as tank, tooo slow and too easy kill in teamfight. Instead of dozens lives, you get a lot of armor and magic resist, also a lot of speed, and probably you are the fastest on the map.

First of all, your number zero skill: For every 1 mana point, you get 0.25 points of lives. This is why Rod of Ages and Banshee´s Veil are so awesome for you.

Poison trail
this is your main killing and farming ability, it does damage over time to everybody inside. Gas does about 22 magic damage at first skill level, 70 at skill level 5. But every time somebody enters, he is poisoned by amount of 5 seconds, so gas does around 100 damage. Little amount of your ability power is counted into this damage, if i remember well, 35%.

Mega Adhesive
is your only ranged ability of spilling pool of glue on the ground. It does no damage, but it has a big range of use and a nice diameter. Useful in teamfights.

Fling
is the best and the most difficult-to-use ability you have. It throws enemy which is really close to you behind your character. It also does a lot of magic damage (at skill level 5 does fling 300 points). Also most of your ability power is counted into it, something like 85%, if i remember it well.

Insanity potion
Your ultimate skill is really ultimate one. Increasing your mana and health regeneration, giving you movement speed, attack speed, attack damage and ability power. With this, you can quickly destroy turret, while there are coming enemies, or finish any cocky carry, which gets into your way. Also it is a good escaping ability, together with gas. It has a low cooldown, after using Insanity potion, recalling back and getting back on the line, you have nearly no time remaining to next use of it.

As Singed, you are one of the strongest farmers and pushers in game. Use your gas combined with basic attack to lasthit minions, that could be killed by your own minions. You should know where will be massive minion wave and get into there before it destroys or damages turret, while your squishy carries heals from the last teamfight. Also you should make sure before each teamfight, that there are no minions which could destroy any of your turrets alone.

Beware of your allies, which want to kill your minions, you already gassed down. For example after gassing 20 minions alone, taking 150 dmg from them comes one stupid Veigar and kills them all with one darkmatter... Make sure your allies don���´t steal your farms, you need them, because you will not get too much kills.

You can also create a big wave of minions, which will push side turret, while you are attacking all in mid. Do it by killing 2 minions in enemy wave, that will hold your minions on place for a while, waiting for reinforcements, but not taking too much dmg. This way you collect minions from three waves, so about 20 minions will come to enemy turret - and if no enemy champ comes to save it, they destroy it.

Cleanse is lifesaving spell, one of your best and most used. You are mad if you don���´t get that one point in masteries into it. That one single point reduces cooldown by 20 seconds, which is soooo much. Beware of Warwick and Malhazar, clense doesn´t work against suppression.

Ghost is your second ultimate ability. Its necessary to have that one single point in masteries in it, it gives another 3% of speed and increases length by 1,5 seconds. Ghost is especially important in turretdiving, making you able to get fling and yourself out enough soon. It also makes you capable of walking through units, including enemy champions. You an run threw them, stop before them, use your basic attack and gas why they are walking around you and when they are finally before you, fling them back to your gas and allies.

I allways start with sapphire crystal and two health potions. If you play safe, you sould survive on the line up to level 6, when you should try to finally get you first blood using your ulti and ghost. Before turretdiving, drink all remaining potions to restore your health. Than you should get boots and ruby crystal, if you did those kills, then you can buy Catalyst the protector. After Catalyst get your Boots of Swiftness. Sometimes i buy only ruby crystal and focus on boots first, do it if you want to try getting kill(s) quickly. This time is good for getting some sightwards.
If you were killed soon, buy boots and if remains you some money, get one sightward and some healing potions.
After getting Boots of Swiftness, you are one of the fastest champions on the map. Use it. In this phase, you need kills. Build Rod of Agesquickly, get it around level 11. If you play good, you don´t need to return back, because "Catalyst the Protector" is healing you with each gained level.
So after building RoA, get another one Catalyst the protector.
Then you should focus on armor or magic resist, but Banshee´s Veil is easily reachable since you already have one part of it, so it is good to start with it, if there is no insane attack damage carry in enemy team.
After building the Guardian Angel, buy Force of Nature, which is the strongest MR item and gives you amazing health regeneration. And also, ot gives you another bonus to speed.
Last item should be Lich Bane. Gives you more of speed, little bit of magic resist, mana and a lot of ability power, which you would be now lacking a bit. Also the sheen ability is quite a useful, after flinging enemy you can deal with ulti and lucky crit 600 dmg, which is after teamfight so much.

If this built has some weakness, it is in low amount of lives. If you play against fed team with many tanks, you need more surviving items. Against Cho Gath, which deals big damage with no penalty for armor or MR by his ulti, get Warmog´s armor, which is granting you 920 lives and health regen instead of Lich Bane, if the team is composed a lot of ad dealers, get Sunfire Cape instead of Force of Nature.
Instead of Lich Bane you can also buy Rylai´s Scepter, which gives you health, ability power and granting slow to all of your anbilities, including gas.

Your gas is most important, so use all your points as quickly as posible into it. You can get Mega Adhesive sooner, if you prefer, it is usefull for teamfights. Since gas is your main farming and killing ability, be sure that you have all points you can in it.

If you want to try to get first blood, then maybe you would find some use in taking fling as first ability. Dont forget, that enemy which is trying first blood as well probably has a lot of lives (Catarina with runes about 800) and sometime also a plan ;-) Your basic attack does the same amount of dmg on first level as their, so use it.

Your part is the most difficult, you are a tank.

If the team lost because your carries are weak, you are the last who remains in there. this mean you are alone against 3-5 enemies. You can try to finish one of them with quickly fling, but then, you must run. Gas your enemies all the time, change direction in bushes, fling your enemies coming from front of you behind yourself. In low level games, you can actually get some kills by just going with your gas... Use your Mega Adhesive to slow your coming enemies, place it under yourself and continue in escaping.

During teamfight, your work is gasing all the enemy team. They will not focus you, at least if they are clever, and if they do, they will lost soon, even if you die. Your work is to save your allies, as well. Fling away meelee fighters from your ranged carries, slow their coming allies somewhere. If you split them well, your team will fight something like 5v3, 5v2 and so. Next thing you must do, is disrupting of ugly abilities like Nunu���´s ulti, after you fling him, it stops. If you are catched in Galio���´s ultimate, use cleanse, stop it by fling quickly. Throwing out fiddlestick from your allies is also good idea.

You are the main battle iniciator, allways enter the batle first, gas your enemies, catch the first carry you find and fling him to your allies, 4 v 1 will be quickly death for him and nice assist for you. Gas the entire team, even if you die, you get 5 assists. Many people think that a player is good if he has a lot of kills, but you are good even if you get just assists, score like 2-5-23 is really good for you.

Be sure you are the one who farms most, after 50 minutes you should have about 200 minions killed, that means about 4000 gold for you.

Singed is after tryndamere, maybe kassa and vlad, the best guy for turretdiving. All the magic is in siple combination of ulti, ghost, cleanse, fling gas and basic attack.
Using ghost gives you a big advantage - you ignore the unit collision. You can gas your enemy, get before him and fling him out of turret. Not mentionning the given speed which is so important.
Ulti increases your ability power, movement speed, regeneration attack damage and attack speed, so is prety important, you can use Insanity potion (Ulti) or Ghost, but best is using them together.
Gas get on while entering close to the turret, you never know which direction will enemy run.
Fling your enemy out of the turret, maybe he become running on the side you flinged him, if he is alone, catch him, and kill him out of the turret.If the enmy runs in the circles around turret, which is btw one of the best ideas, gas him and try to change direction to cath him from other side, if he is too fast.
Minions are important. Never turretdive on low level (before Guardian Angel) if none of your minions are present. Turret becomes attacking you when you do some damage to a hero inside its range, which is giving you 2 or more shots, eaten by minions.
When you turretdive against hero with some disable, like slow, fear, snare, stun, make allways sure you have your clense spell ready. Also heal spell might be usefull, but cleanse is for you better.

There are champions which you shouldn���´t turretdive - fidlesstick, which fear, lifedrain and silence is so bad for you, Kassadin, whose teleport always saves his life - and any tank with charged guardian armor. Also Warwick and Malhazar are dangerous for you, being surrpresed under turret is really ugly, even for you.

Since this is a tank build, get all 21 points into the defense, especially spare 1 point for the cleanse. In Utilities, be sure you get a one point into ghost and 4 points into increasing total mana and regens.
One remaining point might be used for mana regen or increase of your crit chance, as you wish.

Getting a lot of mana from runes is good for you cause singed a little bit sucks with mana at the beginning. Buying Sapphire crystal and getting mana from runes solves that problem. Together with regeneration, not wasting your gas and fling too much, you should save enough mana up to level 6, when you should be able to use ulti, fling and gas for 10 seconds.

Speed is really important, makes you able to catch running enemy before he gets to turret. Also catching your enemy and flinging him to your turret is possible only if you are faster. Or if they are tards... Get boots soon, you need them most.

You probably wondered, why so many crit chance runes. Because they are awesome. Why you turretdive, you need to kill your enemy as soon as possible. With your ulti on, you get bonus to attack damage, so your critical strikes really hurts. When your enemy is already in your gas, attack him with basic attack, run when you are sure that gas will finish him, to avoid getting more damage from turret. Your gas is like ignite, just cheaper. Crit chance runes migh give you great advantage in fight before minions are spawned - every enemy does about the same attack damage as you, but you have crit chance, gas or fling. And when you are loosing, run with your cleanse and ghost away from anything.