Just to clarify, this is not a solo top build, it is for bot lane supporting preferably an AD carry. try to dominate the bushes and throw them back. In my case, i like to play aggressively due to his passive and the regrowth pendent, so don't be afraid to go poke them with roar and flip them if you can.

What you want to do is to cover your ad carry, always stand in between them and your enemies. in lane, try to stay in the bush to pressure them, charge out with rolling thunder sometimes to scare them and hopefully harass, this will cause them to be very cautious and scared and hopefully miss some last hits. if you do happen to flip them, roar right away and attack as you move, if the situation seems in your favor, exhaust them and go for the kill, tower dive if you have to, you always have your passive and summoner heal.

In team fights, always make sure you are the one to initiate. This is critical because his flip acts as a short stun, followed by you slow. Never flip their tank or bruiser because that's just bad, try to go for ap or ad carries. And if your team is smart, the victim will be dead right away. activate thunder claw as fast as you can, because it is usually long enough for a team fight and is not worth saving for the end due to its large chain damage.

Farming is important for any champions. In this case, volibear will become a tanky dps late game, so farm a little early game to get your two gold for 5 items as quickly as possible. and then let your ad carry to have all the farm. once you get your boots of swiftness, all you farm will come from kills and assists. Don't be afraid to get the kill with frenzy, as a kill secured is always a kill assured.

i picked exhaust and heal because they are fit for volibear, they can be used both defensively and offensely.
exhaust and shut down carries in team fights and help you live through tower divers
heal is an excellent spell because combined with your passive, it can suprise the heck out of enemies who are trying to fight you.

http://www.mobafire.com/league-of-legends/mastery-tree-planner#&tree1=1-3-0-0-4-0-0-0-1-0-0-0-0-0-0-0-0&tree2=1-3-3-0-4-1-0-1-0-0-0-3-0-1-3-1&tree3=0-0-0-0-0-0-0-0-0-0-0-0-0-0-0-0&v=2