Blue = Game reference
Green = Stat reference
Yellow = Important areas

This build is good for a good tanky initiator. Although, when he initiates, his teammates must be nearby so that he can have a quick escape. He is good for chasing and slowing enemies early game with his Majestic Roar and his Frozen Mallet after you obtain it. You will be very tanky early game with the Frozen Mallet, and the 40% slow will be very good for your Rolling Thunder. This build is based on getting high slow and damage, while remaining tanky and having a lot of sustain.

You may be wondering, "Why the hell AP and Magic Penet. runes?" Well, these runes are contributive to your Majestic Roar and your ultimate. This is so that you do not have to waste an item slot buying an AP item. The speed quint is for early game speed, since most champions will have the same speed, it'll be good if you have early game speed. It's like having Boots without buying them.
Instead of AP runes, you can always get 9 Armor Penet Marks, 9 Glyphs of Fortitude, and 9 Seals of Fortitude. You do not need to change the Quints.

Masteries are pretty simple. They are just tanky masteries along with AD and AP. Includes CD reduction for more Majestic Roaring.

Items. You need Frozen Mallet for early game tankiness and early game slow. You then get Mercury Treads for tenacity and more Movement Speed. If there are little/no cc on their team, you can get Beserker Grieves. You then get the two Recurve Bows for AS, and build them into a Wit's End and an Ionic Spark. These two items will be good along with Frozen Mallet, as you can get plenty of hits on them while they're slowed. If you're looking to shred through HP, you can get the Madred's Bloodrazor first. If you're looking for more sustain and damage, get the Bloodthirster first.

Pretty simple. Focus Majestic Roar, get ult when you can, max out Frenzy after Majestic Roar, then put the rest on your ult and Rolling Thunder.

If your team is needy for AD and not cc, focus Frenzy, then Majestic Roar. If your team needs more support, focus Rolling Thunder, then balance Majestic Roar and Frenzy.

Summoner spells are simple.

FLASH

You can use flash when you are in danger or to catch up to someone.

HEAL

You can use heal when you are in danger, chasing and tanking a turret, or in a team fight, noticing someone is low HP. You don't need any offensive spells because you are a tank.

PROS

Some pros may be-

Good in team fights.
Good for chasing
In team fights, can isolate champions.
In team fights, can slow all.
Good attack speed = faster Frenzy charge
High HP
Moderate armor and MR
Good farm.

CONS

Some cons may be-

Can be shut down if there is crit range
Is weak to tanky opponents (unless you have Madred's Bloodrazor)
Runs out of mana easily.
Low mana.
Weak to strong ap champions. (I.E. Ziggs, Kennen, Cassiopeia)

Easy farm. Super easy. All you have to do is use Majestic Roar, and your Ionic Spark and attack speed will take over. Sometimes, 2-3 Majestic Roars is enough to kill them all in an area.

Here are some situations I've found myself in.
Main Situation - Say it's a 1v2-4 situation. You're holding a turret. First, you can target a champion, (preferably the squishiest and/or least cc) and activate Rolling Thunder on them. You can fling them into your turret, use basic attack and continue using basic attacks until Frenzy is up. Now, if you think you are, at any time, targeted, fall back behind the turret and play safe. If not, continue on and use Majestic Roar. If they are still not dead, you can use the Frenzy active to do major damage.

Main Situation - Say it's a team fight. Keep using Majestic Roar until someone is low enough for your team to target. Once you find someone, wait until your Majestic Roar is back up, then target them. Use Rolling Thunder and then use Majestic Roar. Keep basic attacking and using Majestic Roar until you see someone low enough to finally kill. When you see someone with about 200-400 HP, Use your active Frenzy (should be up by now) and take them out. During this team fight, you can activate Thunder Claws for area damage. This will help deflate the enemies.

Main Situation - Say it's a 1v1. First, use basic attacks until your frenzy is up and they try to escape. If they are stronger than you, Flip them over, use Majestic Roar, then make an escape. If you are stronger than them, and they try to escape, chase with Rolling Thunder. Then you use Majestic Roar, and they will be slowed enough for you to hit them several times. They will continue to be slowed and have almost no chance of escaping. If they slow or stun you back, use flash or Rolling Thunder to catch up. If at any time you feel that they will over power you, use Thunder Claws for that bunch of extra damage.

RARE SITUATION - If you are chasing someone, and they're one hit away, (not even close, babyyyy!) and there's an enemy minion wave nearby, activate Thunder Claws, hit the nearest minion to the enemy champion, and your ult SHOULD hit them. If it does, they're probably dead if they're that close to dieing. Your ult will do 100-160 per hit to 4 different enemies nearby.

So if you're looking for a tanky, fast, strong melee champion, here it is. Volibear is a great choice because he has tanky attributes and good supporting skills. As well as supporting skills, he has good basic attack skills such as Frenzy, and Thunder Claws. I hope you guys liked my build guide, this is my first one!