I've just recently learned to use BBcodes, and I hope you enjoy the guide more now.
I've also made a few changes in the item build and the rune build, such as removing Frozen Mallet for Banshee's Veil, and having lesser kinds of runes, and more of the same kind.
I would prefer that you comment and vote if you have tried the guide, and please, no down-vote without a comment saying why.
I will try to keep the guide updated and as I learn BBcodes better I will make it look better too.
Feel free to leave a comment with tips on how to use the codes, what to change, what is good/ bad etc.
If this guide is appreciated I will make more guides to some of my favorite champions. Then you can expect a guide for Ahri, Ziggz, Graves and more.
And thanks to jhoijhoi, who helped me with the codes with his "Make-a-guide"-guide.
That's all from me. Have a good day!

TheKut

I use items to make Volibear a full tank. I know that many people don't like to be full tank because you mostly don't get that many kills then, but they are very useful in-game and are very appreciated by other summoners in your team. Volibear is also very good at getting kills even though he doesn't have that much damage and attack speed.
If you get the full guide you can close to always tank two other champs by yourself and chase them away with 1/3 of your health to go. With a few more teammates you can also get the whole team. And if you have to run away from team fights you can join again in a few more secinds due to the combo of Force of Nature, Spirit Visage and The Relentless Storm.

I don't depend that much on runes since I rather buy new champions instead, but I picked out the runes that I use and would use if I could afford at the moment.
These runes are:

• 9 Greater Mark of Armor
• 9 Greater Seal of Vitality
• 9 Greater Glyph of Magic Resist
• 9 Greater Quintessence of Vitality

The things that a real tank needs is health, magic resist and armor, and these runes gives a fair amount of all of this.

Masteries are most important early-game, but even here you should go full tank. You should go, most preferably,

I allways use a Doran's item whatever champion I may choose because of the good stuff you gain from them. And Volibear's Doran's is of course Doran's Shield.

After that it's important to get some speed to make running and chasing (especially with Thundering Smash) more usefull, so get Boots followed with Ninja Tabi because of the armor and the damage reduction.

After that you should get Thornmail. Many Volibear users doesn't get it, and I don't see why. Thornmail is a great item to any tank and due to the passive you can make it in team fights much longer.

You also need to get health pretty quickly, and therefore you should get a Giant's Belt followed with Warmogs Armor as quickly as you can after Thornmail. It's passive can also help you a lot as a tank.

Now it's time for the Spirit Visage- Force of Nature-combo. Those items can help you alot since they buff each other. Force of Nature gives you a good amount of health regeneration every 5 seconds and also gives you extra health regeneration in proportions to your total health every second. And efter you have that the Spirit Visage buffs these effects with 15%. Spirit Visage also buffs The Relentless Storm which makes it even more overpowered.

Though, unless you're up against an AP-poor team, or really dominates the lane I would recommend to buy Banshee's Veil. It's passive can really help you against casters, and the extra magic resist, health and mana can be really helpful too.

If you don't want Banshee's Veil for some reason - like if you, as I said before, dominate or are up against an AP-poor team - you should get Frozen Mallet instead. It's an incredibly good item 1v1, and also gives you a major health boost.

I would probably also suggest Randuin's Omen, but I won't since it doesn't fit that well with the build unless you're up against alot of mages. As you know, mages doesn't use basic attacks that much, hence making the Thornmail close to worthless. Then you should get Randuin's Omen instead.

Frozen Heart will also do just fine instead of Banshee's Veil.

Chosen of the Storm

This is your passive. When you reach 30% of your maximum health it will set off and heal you for 30% of your max health. When it has set off it has 120 seconds cooldown. If it is on cooldown and you go below 30% health and the cooldown finishes, then it will no set off before you get hurt one more time.

(Q) Rolling Thunder

Your first choice ability. It raises your movement speed with 15%, or with 45# if you're running towards an enemy champion. This speed boost lasts for 4 seconds, or until you attack something. When attacking after using this you will get 30/60/90/120/150 bonus physical damage. You will also fling the target behind you unless the target is a turret, nexus, inhibitor, Baron or the dragon.

(W) Frenzy

Your second ability, and your most important skill, except your ulti. You gain 8/11/14/17/20% AS. This effect stacks up to 3 times and gives this ability an active that can be used after 3 stacks. It allows you to bite a target, dealing 80/125/170/215/260 (+15% of bonus health) physical damage. This damage is increased by 1% for every 1% of the targets missing health.

(E) Majestic Roar

Your third ability. It is good while chasing, in team fights, while running and while farming. It deals 60/105/150/195/240 (+60% of ability power) magic damage to all nearby enemy's and slows them by 30/35/40/45/50% for 3 seconds. Minions, monsters and pets are feared too.

(R) Thunder Claws

Your ultimate and final ability. For the following 12 seconds your autoattacks will blast the targets with lightning, dealing 75/115/155 (+30% of ability power) magic damage to the target, as well as 3 nearby enemys, but will only hit each target once. If possible it will focus champions.

The first thing you should get is Thundering Smash. Some summoners use Frenzy instead, and that may be good if your going solo, but in my oppinion you shouldn't go solo at all with Volibear (until you get a bit into the item build at least), and therefore Thundering Smash is better since it keeps your targets from running away.

After that you should get Frenzy . It's a great skill due to the great damage you can deal after a while.

Now it's time for Sky Splitter. It is great in a combo with Thundering Smash since it slows the opponents, preferably after you have thrown them behind you so they can't run, or before to make sure you can make it to your target before Thundering Smash weares out.

Now that you have the tree first abilities you should max Frenzy at level 8 so it cn make more damage faster. You should also - of course - take your ultimate every time you can and max it at level 16. Sky Splitter is also good to have maxed prety quickly. Thundering Smash's pretty much only purpose is to prevent your targets from running and therefore it's not that urgent to max it quickly.

Exhaust This is a good spell at any time. If your opponent runs away you have three ways to keep them near you; Thundering Smash, Sky Splitter and now Exhaust.

SPACE

Ignite This one is really useful with any champion. If you, even with your good CCs, doesn't manage to keep them near you long enough, Ignite is a good way to secure a kill.

Ghost This is a good choice too, especially in combination with Thundering Smash.

SPACE

Heal A good spell to any tank, though not very appreciated by your opponents, or by me. Though, it syncronices well with Spirit Visage and Force of Nature.

SPACE

Surge It gives you a pretty good boost in AP and AS, and it makes you grow.

SPACE

Flash Another spell that can be good with any champion. Can increase or decrease the distance between you and your opponent, and allows you to travel over and through walls.

Revive It's not a bad spell actually, but with another spell like Ghost or Flash you might not have died in the first place.

SPACE

Smite This one is completely off limits. I know that many people thinks that Volibear is a good jungler, and he is, but he's a way better tank, and there's better junglers than him out there, so no Smite.

SPACE

Promote I doubt that this is good enough to make up for the loss of more useful spells, even though you can make alot of gold from it.

SPACE

Clarity I'm sure that everyone that have played League of Legends even just for a while knows that Clarity shouldn't be used with tanks, but I've seen a few tanks with it, and it's not good. It might be good in early game, but with the mana you get in higher levels it won't be needed.

Majestic Thunder

This is well known and useful combo made out of Thundering Smash and Sky Splitter.
You start by charging an enemy - preferably with gank - with Thundering Smash, and fling him/her behind you. As soon as they try to run away you use Sky Splitter to slow them and prevent them even further to run away. If your team is prepared you most likely will get the kill.
Sky Splitter can also be used before you fling the target behind you, just to make sure you reach them within the 4 seconds that Thundering Smash is active.
Exhaust and Ghost is also good attributes to this combo. Ghost is used to increase the movement speed from Thundering Smash and Exhaust is used to make sure that they won't get away.

Last Hitting Frenzy

When ganking, you will most time be able to land more than 3 basic hits, and then Frenzy 's active sets off. That's when you should save it. When the opponent(s) run low on health you should use Frenzy 's great damage to make sure that you can take the last hit and get the kill.
It works most time, but many other champions have similar skills that is good for last hits, so be on your toes.
The same technique can be used if someone tries to steal a buff from you when jungling.

Chosen Heal

This is also a good combo if you have Heal. Just as your The Relentless Storm sets off, use Heal. With these heals together you will be back to full health quickly.
It's a pretty obvious combo, but it's still useful.

The Full Damage Combo

This is a combo that all champions have, and it contains all your abilities. To use this to the fullest you should be able to depend on your tank advantage. Start by making an enemy attack you, thinking they can take you down, the immediately set off Stormbringer. They probably won't run yet, but just use their own ulti themselves. That's were the tanking comes in.
They won't do as much damage as they hoped, but probably won't run just yet. If you think that the fight is too even, try to get the advantage by using Frenzy . The massive damage it will deal will probably make them run, and now it's time for Thundering Smash to make them stay.
It surely won't make them stay, really, but it will give you time to land a few more hits before Sky Splitter slows them and you will keep up and land a few more.
This is most time enough to get a kill, or at least enough to make them think twice before attacking you again.
Though, if they really can match you, you will still have The Relentless Storm to win for sure.

Minion Lightning

This is a simple but useful combo. If an opponent gets away with barely any health left, this is what to do: Chase them until you get near minions, set off Stormbringer, attack a minion, and the chain lighting will focus the enemy champion.
Remember that they have to be really low on health for this to work since they can only be hit once in a row, unless there are 3 minions or less there.

+ One of the best tanks in the game
+ An insanely good passive
+ A good skill set
+ Awesome dance
+ Good CCs
+ Viable escaper

- Really lousy 1v1 early-game
- Item dependant
- Actually kind of boring skills (though, really fun to play)