If you're looking for a balanced class to play, you can pretty much spec anyway you please with a Druid. As a hybrid class, the Druid has lots of flexibility, able to tank, DPS, Melee, and Heal. Like any hybrid, you won't generally perform an ability as well as a more specialized character, but your ability to do a little bit of everything makes you highly effective. Just because you can do everything though, doesn't mean that you should. So it's a good idea to figure out the best Druid leveling build that will allow you to level fast, and we'll do that here.
Best Druid Leveling Build
The Druid has three potential spec trees - Balance, Restoration (Resto) or Feral. If you are looking to get to level 70 fast, then Feral is going to be your best choice. It can grind fast and suffers very little downtime because of the manaless damage so that gives you more time to be out there gaining experience.
Now there are a ton of Feral build choices out there so I'm not going to get into that here. Just do a search on Feral Druid builds and you'll find lots of options and arguments over what the best build should be. The Feral spec tree is probably the most popular for Druids, so you'll definitely discover a lot of opinion on it.
Personally, I think you can pick just about any feral build and be okay, so you should go with what kinds of specs and talents appeal to you. My one bit of advice would be to load up on all the feral specs, even if that means making sacrifices elsewhere. I wouldn't bother with Brutal Impact or Feral Aggression unless you have plans to do lots of PvP. Otherwise, forget about them and spend your points on the rest of the feral tree.
If you want to level fast and do a lot of solo quests, I believe it makes more sense to spec for PvE, especially since Resto is really the best talent tree for the arenas in PvP and that will slow down your leveling speed a bit. While questing is generally the breezier way of leveling, Feral can really grind it out and sometimes you just have to do a bit of grinding to gain your next level.
If you want to get the most out of your Druid, then the Druid Class Guide is going to give you the comprehensive understanding you need to get the most out of playing one of WoW's coolest classes, including how to level your Druid 1-70 in 10 days, the best equipment and specs and how to make more gold as a Druid.
For more fast leveling info for all classes, including your Druid, you will want to take a look at the Zygor leveling guides . With guides for both Horde and Alliance, these are by far the most useful leveling guides available and have a great map mod as well that makes life much easier in-game.